The Phylum Mollusca

The time is now about 600 million years ago and the first molluscs have made their appearance on our world. About 100 million years later, during the Ordovician period, at least six of the seven classes of molluscs represented today were present. Many of these first molluscs were but simple, worm-like animals, having segments similar to what we find in annelid worms and arthropods. These first molluscs crawled about the primeval seas, probing for and eating microscopic bits of food.

The great landmass of Pangaea slowly deposits dissolved salts and other chemicals into the ocean. The first primitive molluscs in these oceans now digest these chemicals and begin to use the nutrients to build themselves protective shelters (shells) against their hostile environment. As Pangaea breaks apart around 200 million years ago, the world's great continents slowly migrate, due to plate tectonics, and we begin to recognize the world continents as they are today.

Over time, the molluscs flourish and evolve to fit newly developing habitats. Fossil records show some groups ("taxa") growing larger, some smaller. Some grow spiny, others, shiny. Some coil tighter, some looser. Some coil left to right while others loose their coil all together. Some even loose their shells completely. Some, like the ammonids, evolve into huge numbers of species, and then mysteriously disappear forever only to be found as fossils. About 400 Million years ago, some of these molluscs, first the bivalves, begin to inhabit the worlds freshwater streams and lakes. It took at least another 300 million years for certain gastropods to evolve to where they were capable of populating all land and freshwater water habitats.

During the last million years, the land and freshwater molluscs have evolved very rapidly; however, some groups of marine molluscs appear to be decreasing in the number of species existing.

Today, molluscs live in almost all parts of the world. From the deepest ocean trenches to high up on our mountains, molluscs have found their niche. The number of living species ranges from a very conservative 50,000 according to Brusca & Brusca (Invertebrates 1990) to 60,00 in Rupert & Barnes Invertebrate Zoology (sixth edition 1994) to 100,000 in Kozloff's Invertebrates (1990). That said, it is quite likely that up to half a million species will eventually be formally discovered (see the article on How to name a Species, for how this is done!), since many environments and the deeper parts of the sea-floor are very poorly known even today! Classification and taxonomy of molluscs can vary widely depending on what school of thought.

One thing does remain constant in all molluscs however; - to survive all molluscs must have moisture. To stay alive, they must keep their soft bodies moist at all times and for some like those which live in hot dry deserts environments, this is done by curling up in their shell, secreting a mucous plug and staying holed up until the next bit of moisture comes along.

The word "mollusc" or "mollusk" (both are correct) is derived from the Latin word mollis meaning "soft". The study of molluscs, "malacology", comes from the Greek word for soft, malacos. The term "conchology" is also used for the study of molluscs; however, it is usually applied to those that study the shell only.

Molluscs, in general, are soft-bodied animals that usually produce an external skeleton (called an "exoskeleton") we call a Shell, which is composed of a limey material: calcium carbonate (CaCO3) The shell serves both protective, and supportive purposes. The one feature common to all molluscs is the presence of a fleshy mantle. This is a fold or lobe (or a pair of them) of fleshy material, which secretes, modifies and lines the shell. Members of all classes except the bivalves possess a ribbon-like set of hooked teeth called radula. These they rasp (think of a fingernail file) back and forth over their food much the same idea as a cat lapping up milk: Vegetarian species use them to scrape algae off rocks and other substrates, while most molluscan carnivores use them to penetrate the surface of their prey - even when that is a decent thickness of shell! In the superfamily Conoidea, which includes the Cones and the largest family in the mollusc world, the Turridae, the radula is specialized into a form of miniature "harpoon", which is used to spear prey, and in many cases, to deliver powerful neurotoxins, to paralyze their hapless victims. Most molluscs have a well-defined nervous system with a primitive brain. Molluscs have a circulatory system and most have a two-chambered heart. Their digestive system usually includes a jaw, pharynx, esophagus, stomach, intestine and anus. They have a reproductive system that produces eggs and/or sperm. Most gastropods and cephalopods have eyes and tentacles.

A phylum is usually defined as group of animals having several features common to all or most of its members. The following features are common amongst most molluscs:

Biologists use various methods for estimating how closely species are related to each other. They look at comparative anatomy, genetics and paleontology (the study of fossil organisms) to help form their theories. Changes within a population (a group of organisms of one species) generally occur due to divergence and speciation.

Divergence within a phylum can occur whenever the population is split into two or more groups with no chance of interbreeding. Divergence is generally brought about by such events as habitat changes or competition for food.

Speciation can result from reproductive isolation (populations can be physically isolated, as in many marine species, without being isolated reproductively, due to larval (veliger) stages which can drift for long distances, thus effectively "connecting" geographically remote populations. On the other hand, behavioral, morphological or reproductive differences in a small segment of a population can gradually lead to reproductive isolation, without much physical segregation. These reproductively separated populations will adapt to different conditions in different regions, via "natural selection" (i.e., survival of the fittest!). They may develop different mating behaviors or breeding seasons, or they may accumulate enough genetic differences to render egg and sperm incompatible. It is due to an accumulation of these changes and other morphological and genetic differences that we have the seven classes of molluscs today. This seemingly advanced degree of differentiation, however, took place hundreds of millions of years ago: by the middle of the Ordovician period, all the shell-bearing classes (6) of mollusc are represented in the fossil record.

Now, let's take an even closer look at these seven classes of molluscs. We'll cover the basic anatomy and physiology and behavior of each group, and a variety of other interesting facts about each. Enjoy!

The two first two classes we will discuss - the Aplacophoran and the Polyplacophoran, are often regarded by some as Subclasses of a larger Class called Amphineura
(Amphineura: (am-phi-neur-a) Latin: amphi =both. neura =nerve).
However, we will follow the crowd (i.e., the majority of scientists), and treat them separately.

(poly-plac-o-phor-a (lor-i-cat-a))
Latin meaning: poly = many plac = plate phor = carry, i.e.: bearer of many plates.

NOTES: 1) There are a lot of rather "technical", or scientific terms in theses class-descriptions. Fear not - most of them will be hooked up to a glossary by the time things are finished, so the reader will just have to click on them to see a definition and derivation. Meanwhile, we suggest a good dictionary J)
2) Images (diagrams and photographs) will also be added to this part of the site, as well as some links to relevant sites.

The polyplacophorans, commonly known as chitons, are often considered by scientists to be the most primitive of all existing molluscs. Strictly marine, the majority of the chiton species inhabit rocky seashore environments where their low dome-shaped shells are well suited to withstanding the violent serge of ocean waves. They all cling tenaciously to the hard substratum and if dislodged from its rock, will roll up into a ball to protect their fleshy under surface. This also allows it to roll around safely in the waves until it can reattach itself to a rock. Most chitons are herbivorous; however a few are predatory. They are nocturnal in behavior.

Many cultures use various species of chitons as food and as fish bait (NOTE: For more of man's uses of Molluscs, see the Man and Mollusc article!)

Chitons are generally bilaterally symmetrical with an ovoid, flattened body. The most distinctive characteristic of chitons is their eight-piece shell. Each of these eight plates is quite similar, except for the first and last (the cephalic and anal plates). The posterior margin of each plate projects backwards, and the anterior lateral margins of each one bears a large wing that projects forward. These projections then fit beneath the plate immediately in front, and each plate overlaps the plate behind: a very tight arrangement - perfect for defense! Except for the posterior edge, a reflexed fold of mantle tissue covers the margins of each plate. In some chitons (such as the genus Amicula), the mantle totally covers the plates. The girdle is very heavy and extends beyond the lateral margins of the plates. The girdle surface may be naked and smooth or covered by scales, hairs or calcareous spines. These so-called hairs can be so long and dense that the animal takes on a mossy or shaggy appearance. Because of the transverse lie and articulation of these plates a chiton can live on a sharply curved surface. If a chiton becomes dislodged (this has not been observed to ever have happened on purpose) from its hard surfaced home, it can roll up into a ball. This could be a defensive mechanism to prevent damaging its softer body parts as it rolls in the surge of ocean waves until it can successfully relocate onto another suitable surface.

Chitons have a broad flat foot, which occupies most of the ventral surface of the animal. It serves both for locomotion and adhesion. Being very sedentary by nature, chitons, especially the older individuals, will stay in a very small area all their lives if an adequate food supply is available. For some species, this could be an area of as small as six square feet.

The foot secretes a small amount of mucous and propulsion is accomplished entirely by muscular contraction.

Both the foot and the girdle affect adhesion. Ordinarily, adhesion is accomplished just by means of the foot; however, when disturbed, the girdle clamps down on the hard substratum and the inner margin is raised. This creates a vacuum that enables the chiton to grip the surface with great tenacity. This is also the reason that chitons prefer smooth hard surfaces on which to live (rocks, shells of other molluscs, lobster traps and other sunken wood, anchors or other metal, etc. Interestingly enough, glass does not make a good substrate, because it is TOO smooth, which makes it difficult to get a truly secure grip.)

Because chitons have ventrally flattened bodies and due to the fact that they adhere to hard substrates, their mantle cavity has had to extend forward as a groove on both sides of their body. This groove runs between their foot and the mantle edge (trust me, this is all a lot easier to picture with a picture, which we'll supply in good time!!). The margins of the Chiton's mantle are held down tightly to the hard substratum making these grooves into a closed chamber. A large number of small, paired gills are arranged within these two mantle grooves. The number of pairs of gills varies from species to species and can even vary within a species itself. These gills hang down from the top, or roof, of the pallial grooves and their tips touch the lower margin of their foot, which divide the grove into a ventro-lateral inhalent chamber at the front of the chiton and a dorsal-medial exhalent chamber at the end of the chiton. As the anterior mantle margins are raised, two inhalent openings are formed through which water flows. This water then flows along the length of the groves through the gills and into the two dorsal exhalent chambers. The two exhalent water currents then converge posteriorly and pass to the outside through two exhalent openings that are created by the locally raised mantle.

The pericardial cavity, which contains the two-chambered heart, is large and is located beneath the last two shell plates. A single pair of auricles collects all the blood from the gills and pumps it to the lone ventricle.

The Chiton's nervous system is very primitive. There is no brain, only a poorly developed ganglia and often that is absent as well (so, while it is certainly unethical to torture a chiton, they aren't capable of feeling "pain" as we know it, and most certainly have no thoughts or emotions of any kind!). Neurons are simply scattered along the length of nerve cords. A nerve ring surrounds the buccal cavity and subradular organ. A posterior nerve ring gives rise to a pair of pedal nerve chords, which innervate the foot muscle. A large pair of pallial visceral nerve chords completes the nerve ring. Some families do have specialized eyes called the aesthetes. These are mantle sensory cells that penetrate the articulamentum of the shell and are lodged within vertical canals leading to the outside of the shell. In some chitons these are only tactile sensory organs. In others they are more specialized - actually taking on the properties of an eye, complete with a cornea, lens and retina! These chitons may possess thousands of these little "eyes" which appear as minute black spots on their shells.

Most chitons are herbivorous - feeding on unicellular and multicellular algae. They scrape this alga off of the rocks and other substrates on which they live, by means of their hard, raspy radular ribbon. The mouth is located at the anterior end, in front of the foot and opens into a buccal cavity containing the radula - and a smaller, more ventral subradular sac that in turn contains a sensory structure. A pair of salivary glands secrete through an opening in the wall of the buccal (i.e., mouth) cavity. When the chiton feeds, the subradular organ is first protruded and held against the surface. If food is sensed, the odontophore with its radula project from the mouth and begin to scrape the algae off the substrate. When the radula is retracted, these food particles are pressed against the roof of the buccal cavity and carried into the esophagus. Saliva is added to this food at this point: it contains no digestive enzymes and acts merely as a lubricant for transporting the food particles. This mucus and food slurry is then carried along the ciliated esophagus towards the stomach. Along this passage, this food is mixed with amylase (a digestive enzyme), secreted by two large esophageal glands that enter the esophagus through two ducts. The esophagus then enters an irregularly shaped stomach that contains a large ventral sac. Further digestive enzymes are added to this slurry and aided with the contractions of the stomach; the food is then churned and dissolved into absorbable nutrients for the chiton. These nutrients are then passed along to the intestine where food absorption takes place. All waste materials pass through a sphincter into the posterior intestine where liquids are further absorbed and solid waste matter is it is compacted and passed along its mucous lined interior. It is then divided into pellets; further compaction takes place until it is finally passed through the anus (which opens at the midline just behind the posterior margin of the foot), as small, solid fecal pellets.

The two nephridia (a kind of primitive kidney, responsible for renal function) are quite large, and extend anteriorly on each side of the body as long U-shaped tubes. They are responsible for removing waste from the blood. This liquid waste is passed out through two nephridiopores into the pallial grove located on each side between the more posterior pairs of gills.

All chitons are dioecious (i.e., they have two sexes). Both males and females possess a single median organ that is located in front of the pericardial cavity under the middle shell plates. Two gonoducts open directly to the outside. A gonopore is located in each pallial groove, in front of the nephridiopore. There is no actual copulation, and fertilization, in most cases, occurs in the mantle cavity: Sperm leaves the male in its exhaled water and enter the female via inhaled water. The fertilized eggs are usually just shed into the surrounding water; however, in some species the eggs are retained inside the female s mantle cavity and she gives birth to the live young that have developed within her oviducts. Since Chitons are gregarious by nature, this form of reproduction is quite successful - and no romance is required.

Latin: mono=one plac=plate phor=carry : bearing a single single plate (shell)

Until the mid nineteen hundreds, this class, often call the "gastroverms", were thought to be extinct and appeared only in Cambrian fossil records. Then in 1952 scientists discovered ten living specimens while on the Danish Galathea expedition. Two years later on the same expedition, two more species were discovered. Now we recognize about a dozen living species.

These strange, limpet-shaped molluscs are segmented like worms. In each segment of the creature the internal vital organs are duplicated. They are indeed primitive in biology and as such tend to live only in the deeper ocean areas where they are away from the more advanced and active predators.

Monoplacophorans possess a single, large, bilateral shell. The shell is a simple depressed limpet or disk -shaped valve, less than 25 millimeters across usually and is often thin and fragile. The earliest developed part is a coiled chamber. The outer surface of the adult is covered with a protective horny periostracum, or sheath. On the inner surface of the shell, there are significant paired muscle scars, suggesting segmentation.

Monoplacophorans possess a foot, round in outline and not very muscular, which is responsible for locomotion. The muscular action is similar to that of the polyplacophorans. (Please refer to the Polyplacophora for info on this.).

Running along the mantle gutter cavity on either side of the body are five or six pairs of gills, however, filaments only exist on one side of the gill axis.

Monoplacophorans possess a single ventricle and two auricles for circulating the blood per body segment. The first pair of auricles receives the blood from the first four pair of gills. The pericardium is paired and the heart lies between the two divisions.

The head of Monoplacophorans is much reduced in size lacks true eyes and tentacles.

The nervous system of this class is very similar to that of the Polyplacophorans. (Please refer to
the data on Polyplacophora for that information!).

The digestive system is very similar to that of the gastropods. (Please refer to that data for details). The difference is that behind the mouth is a curious cluster of frond-like appendages that serve to push the food into the pharynx. Also the mouth is located in front of the foot and the anus is located posteriorly in the pallial groove. From the nature of their radula ribbon of teeth, it can be postulated that they ingest mud or bottom detritus. The radula is actually upside down as compared to the radula of other
molluscs.

Six pair of nephridia are present and are arranged in series (metamerically) on each side of the body. The nephridiopores of the last five pair open near the five-gill pairs.

The sexes are separate, and two pair of gonads are located in the middle of the body. Each gonad is connected by a duct to one of the two pairs of nephridia (kidneys), which are located in the middle of the body.

There is SO MUCH still to be learned about these inhabitants of the deep, and if you become a marine biologist, some day you could be the one to shed light into some of the darker corners or our knowledge of these and other little-studied taxa!

Latin: (A-plac-o-phor-a, from the Latin a=without plac=plate phor=carry: i.e., not bearing a shell)

The Aplacophora are a small group of molluscs, which have deviated from the normal molluscan form. There are approximately 100 known species living today. They are rather worm-like and average about an inch (2.5 cms.) in length. Most live in deep water, except a few more northern species. One group bury themselves into the sand and mud of the ocean bottom where they feed on annelids and other small invertebrates. The rest of the known aplacophorans parasitize hydroids and other corals.

The shell is absent and there is no fossil record to suggest that any members of the class ever had one.

The fleshy mantle does not produce a shell in aplacophorans but is embedded with calcareous spicules, presumably to make them less palatable to predators. Their body shape is slightly oval and flat in appearance; however, in one group of aplacophorans, the order Chaetodermomorpha, the mantle has fused to form a cylindrical body (note: we have Chaetoderms in Nova Scotia - in St. Margaret's Bay (-Ross)!)

Aplacophorans only possess a trace of a mantle cavity. Gills are lacking in many of the species while others possess only secondary gills. The cloaca, a cavity into which the anus and a pair of nephridiopores empty, is possibly a remnant of the mantle cavity.

The foot is either virtually absent or vestigial: a simple ventral fold: It is much reduced and has become just a tiny median ventral ridge lying in a small longitudinal groove. This means that the Aplacophora have no viable means of locomotion.

In the aplacophorans this consists of a simple cerebral ganglion and a lateral nerve cord. There are no specialized sense organs such as eyespots or electrical or chemical sensors.

The head is poorly defined in all aplacophora, and their visceral mass consists of a very simple and straight digestive system. Food taken in passes through the circumpharyngeal muscle into the oral cavity where a radula rasps it (the radula is usually present although it is often somewhat modified). The fine food particles then pass into a single midgut organ that consists of a stomach and digestive gland. A short intestine absorbs nutrients before the waste passes into the cloaca.

Aplacophorans are either hermaphroditic (i.e., self-fertilizing, as in the family Proneomenia) or dioecious (i.e., having two separate sexes, as in the family Chaetoderma) (Two quite different sets of "family values"J). Copulation never occurs: males release sperm freely into the water. The females also release their eggs into the water, or hold them within their mantle cavity. In the latter case, inhalent water drawn into the female's mantle cavity contains sperm, which fertilizes the eggs held there.

Latin: (sca-phoda: from the Latin scaphe=boat , and pous=foot: boat - foot!)

All scaphoda species, a mere 200 or so, are marine inhabitants that live partially buried in sand or gravel all their lives. The tusk or tooth shells, as they are more commonly known, have the simplest shell structure and anatomy of all the molluscs. They show very little variation in structure except that the members of the Dentalium resemble elephant tusks, while those of the Cadulus class look like swollen cucumbers open at each end. The number and shape of the longitudinal ribs along with the coloration and curious slots appearing along the edge of the smaller posterior end of their shell all help to distinguish the various species of tusk shells.

They are relatively inactive creatures with a low metabolic rate and a very simple anatomy. Scaphopods generally burrow in the sand at depths ranging from 18(6 meters) to 600 feet (200 meters), however, a few do inhabit shallower waters and some have even been found in the oceans' deepest trenches.

The western tribes of North American Indians once used tusk shells extensively, first as necklaces and later as money belts (Wampum) to be traded with other tribes, and the white man.

The shells of most species average between one to two inches (2.5 cams.  5 cms.) in length; ranging from 4 mm (Cadulus mayori) to a maximum of about 150mm (6") (Dentalium vernedei Sowerby, 1860). The name "tusk shell" is derived from the fact that it resembles the shape of an elephant's tusk. The shell is usually an elongated, cylindrical tube, open at both ends, and slightly curved. However, some of the scaphoda shells are shaped more like a swollen cucumber. The shell is usually heavily ribbed and has small slits at the narrowest end. Colours range from white & off-white, cream, brown to an attractive variety of greens.

The foot extends from the larger end of the shell and is spade or cone shaped. It is projected downwards, so the animal can burrow with it. It also serves as a means of an anchor for the animal. Finally, by the contracting and expanding motions, the foot also keeps water passing in and out of the posterior half of the mantle cavity which in turn causes blood circulation.

The mantle cavity is large and extends the entire length of the ventral surface of the scaphopod. Water slowly enters this cavity as a result of ciliate action (many small cilia all beating in the same direction). The cilia are concentrated on the vertical ridges of the mantle wall, approximately midway on the animal. The remaining mantle wall is only weakly ciliated. There are no gills for respiration. Gases are diffused directly through the mantle wall. After about 10 to 12 minutes, a violent muscular contraction expels the water from the same inhalent opening.

The circulatory system is reduced to a simple system of blood sinuses (cavities). There is no heart: the foot keeps the blood circulating, as described above, and gases are simply diffused through the cell walls lining these sinuses.

The usual molluscan sense organs such as eyes, tentacle and nephridia are absent in Scaphopods. They do however have the familiar cerebral, pleural, pedal, and visceral ganglia and their corresponding nerve chords common to most molluscs.

The feeding mechanism of the Scaphopods is complicated and unique. Their food often consists of a microscopic family of one-celled test-forming organisms called foraminifer (a good for a site to visit), some of which live on sand or silt, which they glued together to form their tests (which function as shells). The Scaphopod head is reduced to a short conical projection or proboscus, bearing the mouth. They bury themselves head down in the sand or mud - like politicians avoiding an issue!.

On each side of the head are lobes bearing a large number of thread-like appendages called captacula. Each of these tentacle-like food-gatherers has an adhesive (i.e., sticky) knob at its end. The captacula are extended into the surrounding sand or mud to capture food and bring it to the mouth. The buccal cavity contains a well-developed radula with large flattened teeth. The radula aids in ingesting, and shreds their food.

The stomach and digestive gland are located in the middle of the body. The intestine then extends anteriorly, and then loops around to open through the anus into the mantle cavity. Details of scaphoda digestion and absorption are still unknown: they are a complicated lot!! (Want a Masters thesis someday???)

Scaphopods possess a pair of nephridia (renal organs), which drain out of the body through nephridiopores located near the anus.

Scaphopods are dioecious (Two sexes, i.e.). Eggs and sperm reach the outside of their body through the nephridiopores. The eggs are shed singly and are planktonic (i.e., free-swimming). Fertilization takes place when the sperm and egg meet by chance in the water surrounding a spawning couple or group. The larvae drift freely in the water-column until they eventually settle down on the ocean's bottom.

Latin: gas-tro-pod-a: Latin meaning: gaster=stomach pous=foot : stomach - foot!)

Cypraecassis rufa
(by permission of Guido Poppe)
The gastropoda is the largest and most certainly the best-known Class of all the molluscs. They are the most successful of the molluscan classes, and occupy almost every habitat on earth, from desserts to high mountains, fields, forests, lakes, streams and oceans - and most probably your back yard!! It is the only class to contain species that have ventured permanently on to land. (To do this, snails evolved an efficient gliding foot, eyes, an aggressive eating mechanism and a pulmonary system for breathing.) Gastropods also inhabit every niche in the ocean from the intertidal zone to the deepest ocean trenches. Over 15,000 fossil forms have been described and over 40,000 species exist today. They are, scientists theorize, now at the peak of their evolutionary development.

Gastropods exhibit the least change from the ancestral molluscan plan of all the molluscs. The pretorsion (pre = before, torsion = twisting) shell of the ancestral spiraled gastropods resembled a coiled garden hose flat lying on the ground. This plano-spiraled (i.e., coiled all in one plane - flat!) shell was symmetrical. Having each coil lying outside the other was a great disadvantage as it was not very compact nor was it easy to carry around as the diameter could become very great (some fossil gastropods have been found with shells measuring 8 feet (2.5m!!) in diameter - that's heavy-duty hauling!!!).

Then the gastropods underwent a very significant change in their evolutionary path. This change was the twisting or torsion that the body underwent. Most of the body located behind the head, including the visceral mass, mantle and mantle cavity, was twisted 180 degrees counterclockwise (i.e., in a right-handed direction: most species of shell-bearing gastropods are still right - handed, with notable exceptions, such as the Lightning-whelk (Busycon contrarium Conrad, 1840). Many land and fresh-water species and even some entire genera are also left-handed.) Internally, the digestive tract and nervous system were twisted into a U-shape. The mantle cavity, gills and renal and anal openings were now located in the anterior part of the body - i.e., just behind the head.

The problem of the large unmanageable pretorsion shell was solved with the evolution of the asymmetrical coiling of the shell. Now, the coils were laid down around a central axis (called a columella and each coil lay beneath the preceding coil. In order to balance out the weight of this shell, it shifted so that the axis of the spiral slanted upwards and slightly backwards (the asymmetrical part!). The shell was now positioned obliquely to the long axis of the body and the gastropod could move about with relative ease. (Diagram)

Although there are fossil species showing the pretorsion plano-spiral shell, all existing shell bearing
gastropods have this post torsion, asymmetrical shell. Several problems did arise for the gastropod as a result of this torsion however. The main one being fouling . If the water circuit through the mantle cavity had remained as it had been, the anus and nephridia would have dumped directly on top of the head - a dangerous and not too nutritious situation! Sanitation was thus a great a problem with this new shell design, and the gastropods followed three different courses to solve this problem: In one group, this problem was solved by the formation, over the mantle cavity, of a cleft or split in the shell and mantle. At the same time, the anus drew back from the edge of the mantle cavity and moved to a new position just beneath the inner margin of this cleft. The inhalent current continued to enter over the head and pass over the gills, but now instead of making a U-turn, the water current flowed up and out through the cleft in the shell taking with it the anal and nephridia wastes. Some of today s gastropods have retained this primitive cleft or shell slit.

Others modified their internal organ arrangement to have a single-gill arrangement in a new mantle cavity, and developed an inhalant water circuit to the side of the head. .Still another group underwent detorsion , in which the twisting process was reversed and the mantle cavity and anus once again opened posteriorly. A good example would be the common garden slug.

Another consequence of this torsion and new shell position was that it restricted the mantle cavity to one side of the body, and the opposite side of the body was now pressed up against the shell. This compression resulted in a decrease in size, or the complete loss of, the gill, auricle and kidney on that side of the body.

So it came to be that about 500 million years ago, during the Cambrian period, three basically different stocks, with different body-plans arose. Although most of today s gastropods bear a single, asymmetrically coiled shell, some, such as limpets and abalone have a flat saucer like shell. Still others have no shell at all as is found in the sea and land slugs. These shell changes and body adaptations resulted in the Gastropoda being divided into three subclasses:

The typical shell of the gastropod is the familiar conical spire composed of tubular whorls. This shell, which is created, maintained, colored and modified by the mantle, contains the visceral mass of the animal: i.e., all its internal organs. Starting at the apex, the smallest and oldest part of the shell, whorls get successively larger and are coiled about a central axis called the columella, which may be open or closed. The largest whorl terminates at the aperture or opening where the head and foot of the animal protrude.

A shell may be spiraled clockwise (right-handed shell) or anti clockwise (left-handed shell). When a shell is held so that the apex (top) is up and the aperture facing the person, those with the aperture facing to the right are right-handed or dextral and those that open to the left are left-handed or sinistral. Both sinistral and dextral shell can be found amongst members of the some species. (Photo 1, Photo 2). (A good article: "Reverse Coiled Gastropods": by the Jacksonville Shell Club on this subject)

The first shell whorls laid down by the larval gastropod (i.e., while it is in its egg), are called the protoconch (proto = before, conch is shell). It is represented by the smallest few whorls at the apex of the shell, and is usually smooth, and lacks many of the characteristics of the adult shell, often being colorless, or of a different color from the rest of the shell.

A typical gastropod shell is composed of three layers; the outer periostracum, the middle prismatic layer and an inner nacerous layer. The periostracum is thin and composed of a horny organic (made out of protein, actually!) material called conchiolin, which is semi-transparent, being a brown color: the thicker the periostracum, the darker the color, and the more the shell underneath is both protected from sand grains and other abrasive elements of the animal's environment, as well as from acidic water, which some of the hardier gastropods and bivalves can survive in. Shell collectors often dissolve the periostracum of their shells, so they can see the beautiful colors and patterns better. Scientists, however, leave the periostracum on, since it is an important part of the shell. The two inner layers are composed of calcium carbonate. In the middle layer (which we normally think of as the outside of the shell, since it contains the colors and patterns, the calcium carbonate is laid down as vertical crystals. In the thin, inner, nacerous layer, the calcium carbonate is laid down in thin horizontal sheet. Quite often, there are two or more sheets, each of which reflect light differently, creating the shimmering effect called iridescence (actually a product of refraction patterns - ask your physics teacher to explain!) Reserve calcium carbonate is stored in certain cells of the digestive glad and is used for shell repair or to add new growth thus enlarging the shell for the growing animal. Molluscs can only form shells when they can extract CaCO3 (calcium carbonate) from the water, and keep it from being dissolved again. Thus, if a lake or stream is acidic, or the soil is acidic (as in Coniferous woodlands), shells, and therefore the animals that make them to protect and support themselves, cannot survive. Also, below a certain depth in the ocean (which varies with temperature and mineral content, calcium carbonate cannot be deposited, since the water is under-saturated with Ca CO3. This is called the calcium carbonate compensation depth , and no shell-bearing molluscs can survive below this level. To summarize, shells can only be formed in fresh waters that are non-acidic, and in the ocean at depths above the level where the water becomes under saturated with Calcium Carbonate.

Gastropods show an infinite variety of colours, patterns, shapes and sculpturing of their shell (which is why people collect shells, as opposed to the entire mollusc!!). In some gastropods, the shell is only conspicuously coiled in the juvenile stages. The coiled nature disappears with growth, and the adult shell represents a single large expanded whorl. Examples of this are found amongst the abalones (Haliotis), limpets (several families, including Lotiidae and Acmaeidae) and slipper shells (the familiar Crepiduland Capulus). (The limpets became secondarily symmetrical during in their evolution.)

In the family of Vermetidae (the worm shells), the larval and juvenile shells are typical, but as the animal grows older the whorls become completely separate. The adults look much like a corkscrew, and sometimes don't coil at all, forming a tangled mass of tubes called a colony

Amongst many gastropods, the shell has become much reduced or is absent completely. In other cases the foot and mantle are very large and the mantle has reflexed backwards over the shell so that it becomes totally covered. These animals are no longer able to pull their bodies completely into their shells.

The pulmonates show varying degrees of shell reduction and loss, culminating in the slugs (land and sea), which have no shells at all.

The opisthobranchs have a much reduced shell which is closely related to the degree of detorsion they have undergone. A few have well developed shells, such as the bubbles, but most have a much-reduced shell that is often covered by the mantle as is found in the sea hares.

Gastropods are able to withdraw into their shells by means of a retractor muscle. This muscle, called the columellar muscle, arises from the foot and it is inserted into the columella. The most ancient gastropods, the abalones and limpets have two of these muscles but in the more modern gastropods the left muscle has disappeared.

The typical foot of a gastropod is a large flat creeping sole similar to the foot design of the ancestral mollusc. It has become adapted for locomotion over a variety of surfaces.

The limpets have become quite well adapted to clinging tenaciously to the hard substratums (rocks, wood, other molluscs' shells, etc.) where they live. Many marine and fresh water gastropods have adapted to living on the soft sandy or muddy bottom. Others live on seaweed or terrestrial vegetation or under rotting leaves and logs.

Typically, a pedal mucous gland opens onto the dorsal or ventral (i.e., top or bottom) surface of the foot. This secretes a slime trail over which the animal glides. Waves of fine muscular contractions that sweep from the anterior to the posterior (i.e., from the front to the back) of the foot provide the power for locomotion.

The foot of many gastropods bears either a horny periostracum or calcium carbonate disc, called the operculum. This structure is found on the posterior dorsal (the back bottom) portion of the foot. This is the operculum, and it acts as a trap door that the animal can pull shut to close off the mouth of its shell, thus protecting its soft body parts, which are safely inside. The operculum may also be closed tight to guard against dehydration, if it should become necessary. (As in during dry periods or winter (which in many parts of the world is just a dry season), or when a pond dries up!)

Some gastropods, such as the marsh-dwelling pulmonates (Melampus, for example, which can be found by looking at the high-tide mark of a salt marsh, usually on or near the salt-marsh grasses, which are called Spartina), extend the anterior portion of the foot and then pull up the rest of their body behind it.

In another marine snail (Lacuna), the foot is divided into a right and left half by a groove extending down the middle of their foot. This snail moves by advancing one side of the foot then the other side (a bit like walking!)

The pelagic gastropods, the Heteropoda and the sea butterflies, have adapted to a swimming life style. Being pelagic, the foot has become modified into a powerful finlike, swimming apparatus - in many species, almost looking like wings for flying in the water (hence sea butterflies !)

Some of the prosobranch have adapted for burrowing into the soft sand or mud where they live. Here, the front of the foot called the propodium, acts like a shovel. It has also developed a dorsal flap-like fold of the foot that acts as a protective shield for the head. This mode of living is more common in the Pelecypoda (bivalves), however.

A few gastropods are sessile (i.e., once they settle down somewhere, they don't move at all (remind you of any couch potatoes you know??)). They usually attach themselves to the shells of other living or dead molluscs. The foot has adapted to become a sucker-like. The Worm shells are totally immobile and are either attached to other molluscs or entangled in sponges.

Gastropods have developed many methods of attaining oxygen from the water or land habitats where they live. To some extent the exposed body surface, especially that of the mantle, plays a varying role in the respiration of all the gastropods. Most breathe by means of a gill/s (ctenidia) or secondary gill structures.

In the prosobranchs with cleft shells (slit shell, Scissurellidae (which are like mini-slit shells!), and Fissurellidae (the Key-hole limpets) the most primitive type of gill structure and water circulation occurs. In these gastropods there are two primitive gills and the rectum and anus open beneath the shell perforation or cleft, some distance away from the mouth.

In the abalone (Haliotis), the shell contains a row of perforations. The mantle is split along this line of holes. Inhalent water is pulled into the anterior portion of the body by the action of the lateral cilia (tiny hairs) on the gills. The outstretched gills divide the mantle cavity into a ventral inhalent chamber and a dorsal exhalent chamber. Water flows into the inhalent chamber, passes through the gills, then into the exhalent chamber and finally exits through the shell perforations - an ingenious arrangement, actually!

The Scissurellidae have a similar system but instead of having shell perforations, the exhalent current passes out through the long, narrow notch at the posterior mantle edge where the anus exits.

Keyhole limpets (Fissurellidae) have conical shells that either have a hole at the apex or a cleft at the anterior margin (i.e., the front end). The mantle extends through this opening or slit, forming a siphon through which the inhalent water is sucked. This inhalent water then passes over the gill and exits at the posterior edge of the shell opening (so, it is sucked in the top or the front, through a hole of slit, passes over the gills, then exits out the back - it just sounds so much more scientific when said the other way? (NOTE: Actually scientists don't use technical jargon just to impress or confuse non-scientists (although it sometimes seems that way!!), but because each branch of science has developed a more exact, or precise vocabulary than we use in everyday speaking: the work anterior , for example, has only one meaning, which is precisely defined and cannot be confused with any other word - the word front , on the other hand, could either mean the head area, or the part of the animal that is going forward - which might not always be the head area! So, it is better in a scientific context or situation, to use the word anterior : than front , because it only has one meaning, while front has at least two! Half the trick to science is learning how to translate from jargon or science-speak , to ordinary language - hardly anyone ever truly THINKS in jargoneese!!)

True limpets lack the hole or cleft and the mantle have developed an overhang. This overhang forms a pallial groove on each side of the foot. The inhalent water enters from the anterior of the shell, splits into two streams flowing into these two grooves. It then passes through the gills merges into a single stream at the posterior and exits out as a single exhalent stream.

The remaining prosobranchs have undergone major gill structure and water circulation modifications. The entire gill axis is attached to the inner, or body side, of the mantle cavity. They have only a single left gill with filaments that are formed on one side of the axis. Water enters the mantle cavity to the left of the head and exits on the right side. To prevent fouling of the inhalent water, the rectum has become elongated and the anus exits near the right mantle edge in the region of the exhalent water current.

Many prosobranchs have improved on this system by developing a spout-like inhalent siphon formed by a folding of the mantle s edges. Some gastropods even carry this a step further and the anterior edge of the shell has evolved to become a grooved, elongated extension to house this siphonal canal. (A good example of this is found amongst the Tibia shells that have carried this to an extreme in some instances.)

Some prosobranchs have left the water environment entirely. These are the operculate land snails, and are not true pulmonates. They have evolved a lung from the mantle cavity, as have the pulmonates. Most of these prosobranchs are restricted to living in moist tropical environments in order to maintain their fluid balance.

In the opisthobranchs where partial detorsion (untwisting) has occurred, there has been a loss of the original gill structure and a secondary gill has evolved. In the nudibranchs and sea slugs where complete detorsion has occurred, the mantle cavity and gill have disappeared all together. Respiration takes place through the general body surface or through a secondary gill (i.e., a structure which is not really a true gill, but which performs the same function.). To help increase the body surface for this absorption, some have developed numerous projections called cerata. These cerata are usually arranged in rows. Not all opisthobranchs have these cerata however. Their cerata often contain such brilliant colours as red, yellow, orange blue and green, which makes them incredibly beautiful! [link]. Some slugs are smooth while others have developed secondary gills arranged in a circle around the anus. Sea slugs and nudibranchs are amongst the most attractive molluscs one could ever see.

In the Pulmonates, the gill has disappeared totally and the mantle cavity has modified to become a highly vascularized (i.e., it has lots of blood vessels to absorb O2 and give up CO2) primitive lung. The mantle edges fit tight against the foot, except for a small opening on the right side called the pneumostome. Respiration occurs when mantle floor (acting like a diaphragm) tightens and flattens. This causes the mantle cavity volume to increase, which in turn sucks air in through the pneumostome. As the air pressure increases the pneumostome closes, to hold the air in. The muscles of the mantle then relax and arch upwards which increases the gas pressures in this cavity. This increased pressure facilitates (i.e. assists or makes easier) the absorption of oxygen into the highly vascularized mantle wall chambers and also causes the air to be forced out.

Many fresh-water snails are actually terrestrial Pulmonates that have returned to the aquatic environment. Some must return to the water surface to breathe, while others have developed a long retractable siphon from their mantle that they use much like a snorkel (!). Others can absorb oxygen directly through the mantle surface from the water they draw into their mantle cavity. In still another group, the mantle cavity became much reduced and a conical projection from the foot called the pseudobranch developed as a secondary gill.

The gastropods have an open circulatory system - the basic circulatory system found in most molluscs. They have a hemocoel, or open cavity, into which the blood (called hemolymph , for some reason) is pumped. Oxygenated hemolymph is collected from the gills or mantle cavity and pumped into a number of open sinuses. Here the tissues and organs are literally bathed in this oxygen-rich blood. As it passes over the tissues in these sinuses, it flows into the ctendial (gill) vessels where gas exchange takes place. It is then again drawn into the atria to be pumped out of the heart. (Diagram)

The gastropod heart is located anteriorly (i.e., closer to the head) in the visceral mass. In all but the primitive archaeogastropoda, the right auricle has disappeared or become vestigial due to the loss of the right gill - one of the results of torsion, as discussed above. The ventricle gives rise to a single, short aorta, which then branches posteriorly to provide the visceral mass with blood, and anteriorly to supply the head and foot. An enlargement in the anterior vessel - a sort of second heart , functions in controlling blood pressure. Blood from the kidneys usually enters the brachial circulation, but in some cases it returns directly to the heart. In a few Gastropods, such as the family Planorbidae (which you aren't likely to encounter, by the way), the plasma contains hemoglobin instead of hemocyanin. (Hemoglobin uses Iron for transporting Oxygen and Carbon dioxide, while hemocyanin uses copper. Thus, the blood of most molluscs is a light greenish-blue, instead of the red we usually associate with blood, which comes from the iron in hemoglobin).

In gastropods, the nervous system is distinctly ganglionated (i.e., it has well-defined and specialized nerve cells) and is somewhat complex. It is quite asymmetrical, and twisted into a figure eight as a result of torsion.

A pair of cerebral ganglia (which function as a small brain) give rise to nerves anteriorly that connect to the eyes, tentacle and a pair of buccal ganglia. The buccal ganglia innervate (send nerves to) the muscles of the radula and adjacent structures.

A nerve cord extends ventrally from the cerebral ganglion (located on each side of the esophagus) and gives rise to the two pedal nerve cords. These two cords extend to the midline of the foot to another pair of ganglion, which in turn innervates the foot muscles.

Another pair of nerve cords issue from the cerebral ganglia also. These are the visceral (the viscera are the body organs) nerves, and they travel posteriorly (i.e., away from the head) until they finally meet in a pair of visceral ganglia. Between the cerebral ganglia and the visceral ganglia and along the visceral nerve cords lay two more sets of ganglia
(NOTE: so, instead of having one large, complicated brain like we do, gastropods have 8 tiny, very simple brains, which coordinate between themselves, although each pair have specialized functions.) Firstly are the pleural ganglia, which innervate the columellar muscle and the mantle. The pleural and pedal (pedal means foot. remember) ganglia are then joined together by means of a pair of connective nerve fibers. Secondly and more posteriorly are the parietal ganglia. These innervate the gills, osphridia, and the mantle. They send out nerve fibers to the various structures of the viscera as well).

All gastropods display some degree of ganglial concentration. For example the pleural and cerebral ganglia are always adjacent to each other. In many cases the visceral ganglia have been fused together to form a single nerve center. In the genus Haliotis (Abalones), the pedal and pleural ganglia have become fused, and send a long pedal nerve to the foot. In the Busycon (which include the famous left-handed Lightning Whelk), all except the visceral ganglia have migrated forward and are located around the esophagus and just below the cerebral ganglia. Here all ganglia connectives have been lost except those between the parietal and visceral ganglia.

In the pulmonates, even the visceral ganglia have migrated forward, which has resulted in a secondary bilateral symmetry of the nervous system.

Opisthobranchs that have undergone complete detorsion, possess nervous systems that have become symmetrical again, in simple bilateral fashion.

Gastropods possess the following sense organs: eyes, tentacles, osphridia (olfactory organs), and statocysts (organs of equilibrium, like our inner ear: they help the mollusc tell which direction is up - not always easy in water when you aren't on the bottom, or sometimes even when you are).

Gastropods exhibit virtually every type of feeding possible. Members of this class can be herbivores, carnivores, scavengers, ciliary feeders, or parasites. Despite these feeding differences, some generalizations can be made:

The most primitive digestive system is found in the Archeogastropoda: Keyhole limpets feed primarily on sponges that are rasped from the substratum by the radula. The salivary glands in these limpets only secrete mucus, which is used for radular lubrication and food transport. The esophageal pouches are well developed and they as well as the digestive gland produce the enzymes necessary for extracellular digestion.

The area of the stomach nearest the esophagus is partially lined with tough chitin and contains a ridged sorting region. The end nearest the intestinal opening is conical and forms a style sac. A deep groove runs the length of the stomach. The sorting area directs food particles towards the style, where some of these particles pass through the style sac groove. The digestible material is directed and passed into the ducts of the digestive gland. The rest is compacted and passed directly into the intestine. Digestion is primarily extracellular. (Diagram)

Most gastropods are herbivores, carnivores or scavengers. They have lost this primitive chitinous lining, sorting area and style sac of the ancestral digestive system. Digestion has become totally extracellular.

As a result of torsion, all members of the gastropoda class except the Archeogastropoda (they still have both nephridia) have lost the right nephridia, or it has just been partially retained as a part of the reproductive duct. The nephridium is a U-shaped sac and the walls have been greatly folded to increase the surface area for secretion. It is located anteriorly in the visceral mass.

Excretion involves filtration onto the coelom and reabsorption and secretion in the nephridium. The nephridium in most cases drains into the pericardial (peri =around, cardial=related to the heart, hence around the heart ) cavity via a small reno-pericardial canal and the wastes are excreted through a short ureter (nephridiopore).

In opisthobranchs and prosobranchs, the nephridiopore opens at the back of the mantle cavity and the wastes are carried away with the exhalent water. This cannot happen in the pulmonates however, as the mantle is working as a lung. The ureter has lengthened along the right wall of the mantle and it opens on the outside, near the anus.

Aquatic gastropods, like most aquatic invertebrates, excrete ammonia or ammonia compounds. Terrestrial pulmonates convert this ammonia into relatively insoluble uric acid and water. This adaptation helps them to conserve their valuable body moisture. However, they still do lose valuable moisture to the air through their body surface and as a result many can only survive in moist tropical climes of the world. Some, such as the Annulariidae, have developed a small shell-like tube and use this to obtain air when their operculum is closed.

Others have become nocturnal to avoid the heat of the day, or they live beneath moist, decomposing vegetation. During hot dry periods or in the colder months in the temperate regions, they burrow into humus or soil and become inactive (this is called estivation , which is like an extreme form of hibernation, except that metabolic rates approach zero. This means that molluscs in this state can survive many years waiting for favorable conditions to revive them!). They draw the edges of their mantle together and they then secrete a thin protective calcareous membrane in front of their shell aperture. Fresh water snails estivate when ponds dry up and hibernate when they freeze over.

Studies have shown that the digestive gland of most gastropods also plays a roll in the excretion of wastes. Excretory cells in this gland empty out into the stomach and intestine.

The Gastropods are a mixture of dioecious (two sexes) and hermaphroditic (one sex) groups. Most possess either a single ovary or single testis located in the spirals of the visceral mass next to the digestive gland. Often, elaborate courtship rituals proceed the actual mating.

In the more ancient gastropods, the Archeogastropoda, the gametes pass through short ducts and into the right kidney then they pass into the mantle cavity via the nephridiopore. Eggs are provided with, at the most, a simple gelatinous envelope, which is produced by the ovary. There is no need for copulatory organs as fertilization takes place in the open water after the eggs and sperm have left the mantle cavity.

In all the other gastropods the right nephridium has degenerated except for the portion that functions as part of the genital duct. The genital duct becomes considerably longer and undergoes differentiation to provide for sperm storage and egg membrane formation. This longer duct (pallial) leads directly to an opening in the mantle cavity.

In gastropods where the reproductive system provides for tertiary (i.e., tough external membranes which protect the egg better) egg membranes, the males have had to develop a penis so that fertilization can take place prior to membrane formation. This penis is a long extension or fold of the body wall just behind the right cephalic tentacle. The entire male duct consists of a coiled duct from the testis (pl. testes - gastropods have only one.), a short renal portion to the vas deferens and the pallial vas deferens, containing the prostate.

Sperm from the penis is transferred to the female where it is then stored in the end of the pallial oviduct, where the eggs are fertilized. In the female reproductive system, the pallial section of the oviduct has modified to form an albumin (egg white) gland and a large jelly or capsule gland. Eggs are either formed in jelly masses or are enclosed in a gelatinous capsule after fertilization. These eggs then pass through the oviduct to the outside, where they are on their own!

A few prosobranchs, all opisthobranchs, and pulmonates are both male and female (hermaphrodites). The single gonad produces both the sperm and the eggs. The genital duct becomes divided into two channels for the passage of both sperm and egg. Two meeting hermaphrodites often go through complex courtship rituals before the mutual exchange of sperm via penis in oviduct. Fertilization is reciprocal.

Some gastropods, such as the slipper shells (e.g. Crepidula fornicata L., 1758) that live stacked on top of each other, start life as males. Then the male reproductive tract degenerates and the animal then either develops a female reproductive system or another male system. An older male will remain a male if it is attached to a female. The presence of a large number of males will influence certain males to become females. Once female, however, they remain as females forever. (In the world of Crepidula, feminists RULE!).

Latin; bi=two - two plates (Two halves to the shell)
Pele=hatchet pod=foot hatchet foot (shape)

The Pelecypoda, Bivalva or Lamellibranchia (Latin for leaf-gill) (the only class with three names!) is comprised of molluscs known more commonly as just bivalves , because they have two separate halves to their shells. They all have two-part shells, hinged dorsally. The head is greatly reduced in size and their foot is laterally compressed. Their mantle cavity is the largest of all known molluscs. Their gills tend to be very large and not only function for respiration, but aid in food-collecting as well.

Most bivalves have evolved to become burrowers. They have left the hard substratum of their ancestors and have learned to inhabit the massive mud, silt and sand bottoms of our oceans and freshwaters. Some bivalves do however live on, or most often in hard substrata such as clay, rocks and wood. These have become sessile (i.e., once adult, they don't move), or borers (example - the famous shipworms - of various families, including Litihophagidae (litho= wood, phag = eat: wood eater).

Most bivalves are marine, and of these the majority live in the littoral, or intertidal zone. However, some species are found in the deepest abyssal zones of the oceans. Some bivalve species and groups have adapted to living in brackish and freshwater environments. These are found in the five freshwater families the Unionidae, the Sphaeriidae, the Corbiculidae, the Aertheriidae and the Margartitiferidae. Also, some of the true mussels (family Mytilidae) such as the infamous Zebra mussels are also found in brackish and fresh water. Some of the bivalves lead a commensural life style: living with other marine inhabitants, while still others have evolved to become parasites.

During periods of low tide or drought, exposed fresh-water bivalves retain precious moisture by keeping totally inactive (which is called aestivation : their metabolic rate drops to zero, so they can last a long time without water!), retaining fluid within their mantle cavity.

Bivalves have long played a role in feeding the world s population. Another area where they are important is for man's ornamentation and adornment throughout the ages. Pearls are very economically important as a jewelry item, and many bivalve shells are used in various decorative ways. (See the Man and Mollusc article for details on the many interesting uses man has put molluscs, including bivalves to, over the centuries).

Bivalves are highly specialized not only in their shape, but in their physiology as well. Because of this specialization, most remain living in and on soft bottoms such as sand, silt and well-oxygenated mud.

In summary, the taxonomy of the Pelecypods (bivalves, lamellibranches) is a twisted, complex affair, to be tackled at your own risk!

The typical bivalve shell consists of two similar, convex and oval or elongate valves. These valves are attached and articulate with each other. The shell is made up of three layers: The periostracum or thin outer layer that is made of horny, organic material called conchiolin, the prismatic or thick middle layer that is made up of calcium carbonate crystals arranged in vertically, and the nacerous or thinner inner layer that is composed of thin horizontally arranged calcium carbonate crystals. (Diagram)

Dorsally, the shell has a protrusion called the umbo, which rises above the articulation (most commonly called teeth ). The umbo is the oldest part of the shell. The concentric lines found around the umbo are growth lines, which are usually seasonal, making it a lot easier to tell the age of a bivalve than a gastropod! The two valves are attached by an elastic band of cartilage-like material called the hinge ligament, which is made up of conchiolin - the same as the periostracum. The hinge ligament is designed to hold both halves of the shell together. The main muscle of a bivalve is called the adductor muscle, and is also used to hold the shell halves together. When the bivalve s adductor muscles relax, the ligament causes the valves to open. Most species are also equipped with locking teeth or sockets beneath the ligament to prevent lateral slippage. The valves are pulled together through the action of the two strong, adductor muscles. They are antagonistic to the hinge ligament, as just explained. On the inside of the shell is a scar that marks where these muscles attach.

In most bivalves, the valves are similar in structure and size; however, in a few families such as the oysters and jingle shells the upper or left valve is always larger than the right valve. For those bivalves that attach to a substratum, they always do this by their right valve, which is always the smaller of the valves, if there is a difference between them.

In the more ancient bivalves, the adductor muscles are of equal size. Many families have evolved to the point where the anterior adductor has reduced in size and in some, like the oysters, it has disappeared all together. In these cases, the posterior adductor has shifted to a more central location between the valves. (This large muscle is the part most often eaten by man - for example, the round, white meat we often call a scallop is in fact on the scallop's adductor muscle.)

Some bivalves can rapidly shut their two valves. This is to be found in scallops, for example. Here, the adductor muscle is divided into one section of striated fibers and one section of smooth fibers. The striated fibers cause the rapid closing and the smooth fibers sustain the contraction.

The bivalve shell exhibits a great variety of shapes, sizes, surface sculpturing and colours. In size they range from a few millimeters (<2mm in the Sphaeriidae) to over four feet (1.3+ meters in the Tridacna gigas). They come in all colours and colour combinations, and range from smooth as glass to having long sharp spines in their sculpturing (e.g.: the Spondylidae, or spiny oysters .)

The mantle greatly overhangs the soft body and forms a large sheet of tissue lying beneath the valves. The edge of the mantle has three folds; an outer, middle and inner one. The innermost fold is the largest and contains radial and circular muscle tissue. The middle fold acts as a sensory organ. The outermost layer is responsible for secreting the shell. The inner surface of this outer fold lays down the periostracum, and the outer layer lays down the prismatic and nacerous shell layers. The nacerous layer is also secreted by the entire outer surface on the mantle.

The mantle is attached to the shell, in a semicircular line just inside the shell edge, by means of the inner lobes' circular muscle. This attachment leaves a visible scar on the inside of the shell known as the pallial line The pattern of the pallial lines and adductor muscle scars is extremely useful in identifying very similar species, when you only have the shell. This attachment prevents foreign particles form getting lodged between the mantle and the shell. However; sometimes a foreign substance such as a grain of sand or parasite does get in. To prevent this from irritating the mantle, the mantle lays down concentric layers of nacerous shell around the particle. This is how a pearl is formed. Sometimes the pearl becomes totally embedded in the shell itself. Most bivalves are capable of forming a pearl; however, it is the pearl oyster, Pinctada margaritifera that produces the finest natural pearls man uses for jewels. Cultured pearls are started when man intentionally inserts a nucleus (a microscopic globule of liquid or solid irritant) in an oyster. When this pearl is approximately one year old and has a covering of 1 millimeter, this seed pearl is transplanted into another oyster. Three years after this transplant, the pearl is usually marketable.

To facilitate burrowing into the mud or sand where bivalves live, the foot has evolved to become compressed and blade-like. In the more primitive Protobranchia the foot has a flattened sole on its foot. The edges of this sole fold together to form a sharp edge. It thrusts this sharp-edged foot into the sand or mud then it opens it up again so that the foot now acts as an anchor and the remaining body is pulled down into the soft substratum. Some of the other bivalves, without this flat sole, can inflate the leading edge of the foot: it then acts as an anchor and is used for digging-in in much the same fashion.

Bivalve foot movement is accomplished through a combination of changing blood pressures and muscle action. Attached to the shell, just below the anterior adductor muscle is a pair of protractor muscles that extend from each side of the foot and attach to the opposite valve. Blood engorges the foot, increasing its blood pressure. This increased blood pressure in conjunction with the pedal protraction muscles (the muscles which manipulate the foot), causes extension. Withdrawal of the foot is effected by the contraction of a pair of posterior retractors also attached to the foot and shell, and by the contraction of muscle fibers in the foot itself: in other words, the bivalve sort of inchworms its way around by contracting and expanding its foot muscle, thereby withdrawing and extending it using a combination of blood-pressure changes and muscles.

Some bivalves, such as the Cockles (Cardiidae (Cardium)), move along the bottom by means of jumping. Here the foot is extended then contracted violently, moving backwards in the process. Thus, the foot acts like a spring always kicking the animal slightly forward.

The sessile bivalves, such as the oyster and jingle shells (Anomia), have a greatly reduced foot. Scallops also have a reduced foot and swim in jerky, but often quite effective in the short run, movement through the water by slamming shut their valves. This sudden closure causes two streams of water to be expelled rapidly from each side of the hinge, causing a form of jet propulsion .

The mussels (Mytilidae) live attached to rocks, shells, man-made structures such as piers, or other mussels. They stay attached by means of strong horny threads called byssal threads (Diagram). A gland in the foot of the mussel produces these threads. This gland, which is situated just above and behind the small round foot, produces a secretion that flows down the back side of the foot and out to the tip of the foot, which is in contact with the hard substratum. This secretion runs onto the substratum where it hardens as it comes in contact with the water. A thread is formed. The foot then withdraws and this process is repeated many times on a slightly different area of substratum. A web of byssal threads thus holds the mussel fast.

The bivalve body has become greatly lengthened dorsal-ventrally (i.e., it has been flattened), and this flattening, in combination with an overhang of the shell, creates an extensive mantle cavity. The mantle cavity extends anteriorly (i.e., to the front), and to each side of the body.

In some primitive Protobranchs, inhalent water enters the mantle cavity anteriorly, passes over the gills and exits posteriorly. Since these bivalves live buried, sediments get pulled in with the inhalent water. Cilia lining the mantle cavity, foot and gills sweep these sediments to the mantle edge where it accumulates. Every now and then the valves contract rapidly and flush these sediments out. Some, such as the Nucula, also have hypobrachial glands for consolidating the finer sediments that pass through their gills.

Drawing sediment in with the inhalent water was a major problem for the burrowing bivalves and the solutions made by the primitive species in the Protobranchia group was not very efficient. More advanced bivalves adopted several fundamental strategies to overcome this problem: In all the bivalves as well as in most of the Protobranchs, the inhalent current returned to the posterior end. Water enters posteriorly and ventrally, then makes a U-turn through the gills, and passes back out posteriorly and dorsally. This enables the bivalve to burrow their anterior end into the soft substratum, leaving just the elongated inhalent posterior end protruding through the sediment, clear of excessive sand, mud or silt.

The second change came about with the sealing off of the mantle edges where openings are not necessary. This in turn led to the development of the inhalent and exhalent siphons. The mantle edges surrounding these fused edges are often elongated to form actual tubular siphons of varying lengths. This system is very advantageous, as the animal can now remain buried in the sediments with only the tip of its siphon protruding. The siphon can also be retracted by means of the siphon retractor muscle that was derived from muscle tissue of the innermost mantle fold. (The pallial sinus markings on the shell show where this siphon was to be found.)

There is a lot of variation to be found amongst different bivalves as to the size, length and shape of this siphon. Some are short and poorly developed while others are very long and so big that they can no longer be contracted into the shell. Some species have inhalent and exhalent siphons the same length while in others they are quite different. In length

In some, the mantle fusion has been carried to a point where only three apertures are now present. One aperture for each of the inhalent and exhalent siphonal canals and one for the foot. A few have a fourth aperture through which the byssal threads pass.

Most bivalves have one pair of long gills that separate the mantle cavity into a ventral inhalent chamber and a dorsal exhalent chamber (also known as the suprabranchial chamber, for those that like long, fancy words!!).

Cilia provide the power to bring water into the inhalent chamber. Sediment that enters with the inhalent water gets trapped on the lateral cilia and is swept by the frontal cilia to the midline. It is then moved anteriorly (towards the front) along the gill to the foot (which is, ironically, in the front of the bivalve!!). The sediments are then deposited along the ventral (i.e., bottom) mantle edge. Periodically the valves snap shut, flushing these sediments out. Fine sediment particles, however, can pass through the gill and they become trapped in the secretions of the hypobrachial glands located over the exhalent chamber.

In the primitive order of Protobranchs (Diagram), the gill is not folded and palpal proboscides are frequently present. This is the case in the Yolida, Solemya, Nucula, Nuculana, and Malletia.

Other ancient bivalves appear to have a double set of gills. The second sets of gills actually arise from a folding of the single gill. This is the case in the Filibranchia and Eulamellibranchia, where the gills have also taken over the function of obtaining food. To do this, the gills have many more and greatly lengthened cilia on the gill surface. These long cilia projected somewhat arterially and then become slightly flexed (bent) downward in the middle. At the angle of the bend, an indentation or notch is formed. The notches on the adjacent cilia all line up and form a food groove that extends along the underside of the gill. As they developed through time, this flexion increased until the cilia became U-shaped. Cilia on both sides of the axis, now folded in two, became known as the ascending limb and descending limb. This transformed the single gill into a pair of gills in these bivalves and they became known as the demibranchs.

In the Filibranchia, bars of tissue, called interlamellar junctions, grew between the two limbs of each U at intervals. However, the adjacent cilia still remained attached only by tufts of cilia. Each gill was now composed of two lamellae and formed a tight mesh.

In these bivalves, the frontal cilia carry the food particles, which are trapped on the gill surface, downward to the food groove, and the lateral cilia move the water through the gills. Between the frontal cilia and the lateral cilia, along the angles of the gill limbs, a new ciliary tract was formed of lateral and frontal cilia. These cilia prevent large sediments from clogging the gills.

Inhalent water entering the posterior (back) end of the animal enters the inhalent chamber. It now flows between the filaments and moves up between the two lamellae. From the interlamellar spaces, the water flows into the exhalent chamber and then flows out through the exhalent opening. In this system the hypobrachial glands became unnecessary as only the very finest of sediments ever pass through the tightly meshed gills. They eventually disappeared all together.

The order filibranchia with this gill structure includes the mussels (Mytilus and Modiolus are the mussel genera most commonly eaten), and Ark shells (Arcidae) the oysters- Ostrea, Crassostrea and Spondylus; Anomia; Lima; and the scallops (Pectinidae), and the boring Lithphagia.

In the Eulamellibranchia (Diagram), the union of filaments even developed further and the ciliary junctions were replaced by actual fusion. The lamellae now consisted of solid sheets of tissue. The number of interlamellar junctions also increased. They now extend the length of the lamellae dorso-ventrally (i.e., from top to bottom, vertically) dividing the interlamellar space into vertical water tubes. The tips of the ascending limbs have become fused with the upper surface of the mantle on the outside and the foot on the inside. This now morphologically separates the inhalent chamber from the exhalent chamber. Instead of blood oxygen diffusion occurring through the lamella, the blood is now carried through the lamellae in vertical vessels that course within the interlamellar junctions. (And if you can visualize that tangle of concepts, you are pretty smart!!). Water in the inhalent chamber now circulates between the ridges, and then enters the water tubes through numerous pores (ostia) in the lamella. Oxygenation takes place as the water flows dorsally through the tubes. The water then flows into the exhalent or suprabranchial cavity and out the exhalent opening.

This system was improved upon by many of the Eulamellibranchs. In these bivalves the surface of the lamella has been increased by folding. Their gills now have an undulated appearance.

The order of Eulamellibranchia with their gill filaments morphologically fused includes the Cardiidae (Cockle shells!); the edible Mercenaria (Quahogs); the boring clams- the Petricola (false Angel Wings), Hiatella, Martesia and Teredo (ship worms); the razor clams- Tagelus and Ensis; the little Donax clams; Abra; Pholas (True Angel wings!); Lyonsia; Macoma; the most common freshwater clams- Unionidae; Lampsilidae, Anodonta, and Simpsoniconcha; and the freshwater Sphaeriidae and Magaritidreidae.

In all bivalves, the inner mantle surface plays some role in oxygenation. In the Septibranchia (Diagram) (which includes the Poromyidae, and the little Spoon Clams - Cuspidariidae, however, the gills have degenerated and modified to become a pair of perforated (full of holes) macular septa that separate the inhalent chamber and the exhalent chamber. Muscular contractions of this septum move it up and down, which causes water to flow into the inhalent chamber and forcing it out the exhalent chamber. The mantle has in this order taken over the function of respiration completely.

In most bivalves, the heart folds around the rectal portion of the digestive system so that the pericardial sac engulfs the heart as well as a short portion of the digestive tract. The thin-walled auricles are attached to the muscular ventricle that surrounds the rectum. Ventricular contractions are strong and usually quite slow (approximately about 20 per minute). Bivalves exhibit a typical molluscan circulatory route through the heart, tissue sinuses, nephridia, and gills. Minor variances do exist among the different orders and families, but I will not go into these.

The blood is similar to that of the gastropods; however, some such as the Arcidae (Ark Clams) and Limidae, hemoglobin rather than hemocyanin is present - so these have red, as opposed to the clear or greenish blood most molluscs possess.

Bivalves possess a bilateral and relatively simple nervous system. They have three pairs of ganglia and two pairs of long nerve cords.

A cerebropleural ganglia is located on both sides of the esophagus and they are connected by a short commisure across the top of the esophagus. From these ganglia two nerve cords travel to a pair of closely adjacent visceral ganglia located beneath the posterior adductor muscle. Now the second pair of nerve cords pick up and carry the nerve signal to a pair of pedal ganglia located in the foot.

Most bivalve sense organs are located in the margin of the mantle. Many species possess pallial tentacles, which contain tactile and chemoreception cells. The entire margin may bear tentacles with, or without eyes (e.g. Pectinidae and Limidae) but usually these are restricted to the inhalent or exhalent aperture or siphons or often they fringe the pedal aperture.

A statocyst is generally found near or embedded in the pedal ganglia. This statocyst is a small organ of balance and generally consists of a fluid-filled sac containing statoliths (little stones) that help to indicate relative position.

In some bivalves, ocelli (small simple eyes) are present along the edge of the mantle or on the siphons. In the Spondylus and Pectinidae, the eyes are quite well developed consisting of a cornea, lens and retina. These eyes most likely cannot form a well - focused image but they can detect changes in light intensity with the photoreceptor cells found in the ocelli.

Bivalves also possess an osphradium, or chemoreception organ which lies directly bellow the posterior adductor muscle in the exhalent chamber. How this sense organ works is not fully understood as yet (another thesis topic for you!!)

Most bivalves are ciliary feeders (or filter-feeders). Their gills have taken over the role of trapping food particles as well as respiration.

However; in the ancient order of Protobranchs, the role of food collection is carried out by the elongation of their mouth structure, which is formed into a muscular proboscis and a pair of palps that extend back towards the gills. This proboscis extends into the surrounding mud or sand and organic detritus is drawn in and carried along its length by means of ciliary action. It is then passed to the palps where it passes through the two lamellae. Here the detritus is sorted and particles for digestion are sent on to the mouth along a deep oral groove. Rejected particles are swept to the edge of the lamellae then transferred to the mantle cavity along with the water current.

Food entering the mouth is passed anteriorly to the stomach via ciliary action. The stomach is surrounded by a large digestive gland and is divided into two regions. In the first region (dorsal) the esophagus and ducts of the digestive gland enter and it contains a ventral style sac. This dorsal portion of the stomach is lined with chitin except for the large folded and ciliated sorting region, into which the digestive gland opens. At the apex of the stomach is a tooth-like projection called the gastric shield, which arises from the chitinous girdle. At the end of this region is the cecum.

Food is passed along the sorting region of this dorsal section. A few food particles do enter the digestive gland; however most are passed onto the cecum. When the food particles pass out of the cecum, they get enmeshed in great masses of mucus that fills the ventral style sac. This mass is rotated by the cilia lining the style sac, and along with the muscular action of the sac, this mass is moved dorsally into the upper region of the digestive tract. The leading edge of this mucus mass is wound around the tooth-like gastric shield and is pressed hard against the chitinous girdle. This winding process causes pieces to be broken off and ground up. Smaller bits are passed to the digestive gland and coarser bits are passed venrally into a deep groove along the anterior wall of the style sac and they are then passed directly into the intestine. (Diagram)

The ducts of the digestive gland are ciliated and are divided into an incurrent and excurrent tract. Food particles enter the tubules of the digestive gland via the incurrent tract. Here the particles are engulfed by the cells of the gland and are digested intracellulary. Wastes are dumped into the excurrent tract and are moved by ciliary action back to the stomach where they then get swept into the style sac groove and intestine. The long intestine loops once or twice around the stomach and then passes through the anterior adductor muscle and becomes the rectum. The rectum extends through the heart and pericardial cavity and then opens through the anus at the posterior of the suprabrachial cavity. The intestine only serves in the role of forming feces - no absorption takes place here. Feces leaves as well formed pellets with the exhalent water current.

In the Filibranchia and Eulamellibranchia, the gills have assumed the function of food acquisition. The proboscides have disappeared but the lamellae have been retained. These bivalves have adapted to eating small phytoplankton and very little coarse material ever reaches the stomach.

Plankton gets trapped in mucous that is on the gill surface and cilia sweep this mass into the food groove, (some bivalves have both a dorsal and a ventral food groove) which runs along the gill. The food is then sorted in the lamellae and, acceptable food is passed into the mouth and rejected materials are swept to the ventral edge of the mantle and then posteriorly where they accumulate behind the inhalent aperture. When the valves periodically close, these wastes and water are forced out the inhalent siphon.

The acceptable food particles are fine enough that they don t require as much grinding and the girdle of chitin has become much reduced. The style sac and the mucous in this group has consolidated to form a very compact, and often a very long rod called the crystalline style (usually about one inch in length those of the Tridacna or giant clam may reach a length of 13 inches). This crystalline style in addition to producing its protein matrix also produces amylase for digestion but basically it acts very much like that to be found in the gastropods. The projecting tip of the style is rotated by ciliary action and as grinds against the gastric shield the enzymes are shed into the food particles. (The style is constantly replaced at its base and it may spin as rapidly as 11 to 70 times per minute; this rate is affected by temperature, PH, and food pressure as well as the ciliary action.) This mix passes through the sorting area of the stomach and the finer particles are moved into the digestive glands, of which there are from two to twenty. Here digestion and absorption takes place intracellulary. Any rejected or waste particles from the digestive gland are passed directly into the intestine.

The Septibranchia, which have lost their gill structure, have become either carnivorous or scavengers. The pumping action of their septum provides sufficient negative pressure to pull in small animals. These animals are seized by the much reduced but very muscular lamellae and are passed into the mouth. The stomach is lined with chitin and it acts as a gizzard, crushing up the animal. The style is also much reduced in this group and may only function in coating harder particles with mucous to protect the intestine from injury.

In the bivalves, one oddity does exist, the giant clam Tridacna gigas. This species, besides its regular food procuring and digestive process, literally farms unicellular algae of the family Zooxanthellae, which it encourages to grow within its mantle tissue. Some of these algae get engulfed and are subsequently digested by phagocytic cells thus providing an additional food source for the Tridacna.

Bivalves posses two nephridia, which are located beneath or just slightly posterior to the pericardial cavity. The nephridia are folded to form a long U. One arm is glandular and opens into the pericardial cavity. The other arm forms a bladder and opens through the nephridiopore at the anterior of the suprabrachial cavity.

The majority of bivalves are dioecious (two sexes). Their two gonads are very closely situated next to each other and they encompass the intestinal loops. The gonoducts are very simple as there is no copulation amongst bivalves.

In the Protobranchs and Filibranchs, the gonoducts opens directly into the nephridia and provide for the exit of sperm and eggs.

In the Eumellibranchs, the gonoducts opens directly into the mantle cavity very close to the nephridiopore.

A few bivalves such as the Cockles (Cardiidae), Poromyidae, a few of the oysters and scallops (Pectinidae), some of the fresh water clams Sphaeriidae and Unionidae are hermaphroditic (one sex).

In most of the bivalves, sperm and eggs are released into the surrounding water where fertilization occurs. The eggs and sperm, which were deposited into the suprabrachial chamber, are swept out along with the exhalent current.

In a few of the bivalves, such as the common oyster Ostrea edulis L., fertilization occurs within the suprabranchial chamber itself when sperm is drawn in along with the inhalent current. The fertilized eggs then develop in the gill filaments.

In some of the freshwater hermaphrodites, self - fertilization may actually occur in the genital ducts before the eggs are deposited into the suprabranchial chamber. The eggs then travel into the water tubes of the gill and there they develop into larvae.

Latin: ceph=head poda=foot: "head foot" (because the head and the foot are almost at the same place.)

The cephalopods are an ancient and very successful group, some of which are now the most advanced of all the invertebrates. They have long been among the dominant large predators in our oceans. Two groups of cephalopods exist today: The Tetrabranchia (Nautiloidea) with a few species of pearly nautilus and the Dibranchia (Coleoidea), containing the squids, cuttlefishes, octopods and vampire squids (genera, anyone?).

The Tetrabranchia possess an external shell and two pairs of gills. The Dibranchia either have a much-reduced internal shell or it is missing all together and they possess a single pair of gills. From an evolutionary standpoint, the cephalopods appear to be a diminishing group, which is odd, considering the extreme state of intelligence of some species! There are only about 400 species in existence now, compared to 10,000+ known fossil species.

On the whole cephalopods are adapted for a free-swimming existence. Some cephalopods such as the octopus, however, have largely given this up for a less active bottom dwelling life, although many of these are active hunters and scavengers.

Cephalopods are the most active of all the molluscs and can even rival the fish for swimming speeds. Although there are relatively few species of living cephalopods, they inhabit a great variety of habitats in all the world's oceans.

Most cephalopods have a sac that produces brown or purplish-black ink used to ward off attacking fish or to serve as a protective smoke screen.

Cephalopods have attained the largest size of all the molluscs. Most range between a few inches (5 cms.) to several feet (2- 4 meters) in length; however, the giant squids (Architeuthis) can attain a total length of about 55 to 60 feet (20+ meters), although the possibility of far larger individuals lurking in the ocean's depths has always been the subject of sea-lore, legend and speculation - these are the famous, greatly-feared sea-monsters called Kraken by the Norse, and various respectful names in many other cultures. Individual species, especially the squid, are often very abundant and provide major targets for marine fisheries. The chalky cuttlefish bone is also used for a calcium supplement for birds and at time used in making toothpaste.

Only the tetrabranchs (nautilids) produce an external shell in this class of molluscs. Externally, the shell of the nautilus is creamy white with broad reddish-brown stripes. Inside it is brilliant, iridescent mother-of-pearl. The nautaloid shell is very complex, chambered and spiraled over the head of the animal. Even though coiled, it is radically different than that of the gastropods, being divided by transverse septa (diagram) creating internal chambers. The animal only inhabits the last chamber. As the animal grows, it periodically moves forward, and the posterior part of its mantle secretes a new septum. The septa are perforated in the middle. Through this opening, a chord or tube of body tissue called the siphuncle extends from the visceral mass of the animal to the tip of the most distal chamber. The siphuncle secretes a gas into these empty chambers, making the shell more or less buoyant allowing the animal to rise up or sink down further in the water column, as the animal needs to.

Not all fossil cephalopods have coiled shells, nor are the shells of all coiled species similar in shape to that of the nautilids. The cephalopod ancestors probably had a straight shell shaped like a cone as is shown in the earliest fossil records. The most famous fossil nautilids are the Ammonites, and many people collect and pay surprisingly high for their remains, many of which still bear some of the original shell material, preserved for up to 200 million years!! Some of these exceed 8 feet (3 meters) in length with an opening of one foot (31 cms.) in diameter. In some species the shell is so loosely coiled that the whorls are often unconnected, although most are compactly coiled. The largest coiled fossil belonged to a critter with the unwieldy moniker of Pachydiscus seppenradensis that measured up to a little over 8 feet (just under 3 meters) in diameter. The smallest yet found is less than an inch (2.5 cms) in diameter.

The argonauts produce an egg case that many people confuse for a shell. (See reproduction section for more on this brooding chamber).

External shells are absent on the Dibranchia. The squids have an internal pen the octopus have lost the shell all together (homologous with shell). The oldest known Dibranchia are the fossil Belomoids. The internal shell here was much thickened at the apical and lateral walls and in some the anterior dorsal wall projected over the viscera.

In the Spirula, a common worldwide deep-water squid, the shell is internal. It is a loosely coiled tube divided by septa into small chambers.

The squids are highly specialized at swimming, and bear a pair of posterior, lateral fins that act as stabilizers. The mantle contains both longitudinal and circular muscle fibers. On inhalation, the circular muscles relax and the longitudinal muscles contract enlarging the mantle cavity. This creates a suction and pulls in water through the space located between the anterior edge of the mantle and the head. When the mantle cavity is full the increasing pressure created by the water causes the circular muscles to contract that in turn closes the opening through which the water entered. The longitudinal muscles then contract and force the water through the ventral tubular funnel. The force of the expelled water as it leaves the funnel propels the animal in the opposite direction. This funnel is also quite mobile, allowing the animal to maneuver either backwards or forwards. Speed depends on how forcibly the water is expelled. Squid are able to hover or dart away very quickly. In normal swimming, the arms are stretched anteriorly and held closely together. Their fins are extended and they undulate gently. These fins wrap tightly against the body during rapid swimming. When escaping from predators such as large fish and dolphins, squid often jump right out of the water, and some species can travel surprising distances in these jumps, their lateral fins acting as little "wings"!

Nautilus can also swim with surprising speed. The process for this action is the same as that in the squid; however, the ejection of the water is produced when their body and funnel muscles contract rather than those of the mantle. Nautilus often rest on the bottom with their tentacles forming a stabilizing platform. Whether swimming or resting, their gas filled chambers keep their shell upright at all times. Scientists have yet to discover just how the nautilus regulates this gas production: perhaps you could be the one to discover this!

The octopus is built for a more sedentary life style. Their body is globular and bag-like and it has no fins. The mantle edges are fused dorsally and laterally to the body walls producing a much smaller aperture into the mantle cavity. Octopus are able to swim as the squid do but in a jerkier fashion; however, they prefer to crawl about the rocks and crevices on the ocean floor. Their arms, which are studded with sucker discs, are used to pull the animal along or to anchor it to the substratum. Some octopods, the Vitreledonellidae, have returned to a swimming existence. Their arms have become webbed and look somewhat like an umbrella. These web arms are then used similarly to how a human swimmer would use his arms while doing the breaststroke.

The circulation of water through the mantle not only produces the power for swimming, but it provides oxygen for their gills. The Tetrabranchia have four gills and the Dibranchia have two. The surface area of cephalopod gills have been much increased by a type of folding (more info needed here), and are not ciliated as in other molluscs. These cilia are unnecessary, as cephalopods are predators not filter feeders. The circulation of water over the gills is the reverse of what it is in the gastropods. Since water leaves the mantle cavity by means of the funnel, the exhalent current is ventral to the inhalent current.

The circulatory system is largely closed in all cephalopods. It has an extensive system of vessels making it the most complex and effective system of all the molluscs, enabling them to be much more intelligent and able to move rapidly and over extended periods of time.

The circulatory system of an octopus is closed and consists of one systemic heart and two brachial hearts, two brachial glands (gills) and the blood vessels.

The two branchial hearts, which sit at the gill base, collect unoxygenated blood from all the body parts. They then contract and send this unoxygenated blood coursing throughout the capillaries of the gills. The two auricles of the heart then drain the now oxygen rich blood from the gills and pass it into the medial ventricle. The ventricle then pumps the blood out to the body via an anterior and posterior aorta, and eventually through smaller vessels and finally into tissue capillaries.

Cephalopods have a cartilaginous brain case and a well developed nervous system. They exhibit complex and very advanced behavior, which includes problem-solving and even curiosity! (See article Octopuses are Smart Suckers!)

Octopuses have the most complex brain of all the invertebrates. They have long and short-term memories. They learn to solve puzzles quite quickly. Once learned, they can rapidly solve the puzzle upon repetition and they remember how to do it in the future.

All the dibranches (i.e., octopi, squid, etc.) have complex image-forming eyes that are comparative to a human's; focusing is done, however, by moving the lens in and out rather than by changing its shape as in the human eye. (Diagram)

The eyes of the Tetrabranchs (nautilids) are much less complex. They are simple open pits at the end of short stalks. They lack the complex image-forming apparatus of the other cephalopods.

The presence of the pedal and brachial nerve ganglia innervating the tentacles of the cephalopods is direct evidence that the tentacles of cephalopods are homologous to the foot in other molluscs.

Tactile sense in all cephalopods is quite well developed. Their sense of touch is most acute at the rim of each sucker. Studies have shown that even when blindfolded, an octopus can differentiate between objects of various sizes and shapes, often with remarkable accuracy: Some can even detect size differences in spheres, for example, which most humans can't!

Statocysts (organs of balance) are found in both tetrabranchs and disbranches. They are large and particularly well developed in the dibranchs and are located imbedded in the cartilage present on each side of their brain.

When threatened, they will often try to escape by releasing a cloud of purple-black ink (this ink is toxic) to confuse the enemy. The octopus can release several jets of this ink before its ink sac is empty.

All cephalopoda, except the Nautilus have chromatophores imbedded in their skin. The rapid expansion and contraction of these cells cause coloration changes to occur in the skin. Tiny muscle cells cause the chromatophores to flatten out and widen out to a flat plate when they contract. When these fibers relax, the chromatophores close and the pigment becomes concentrated. Some species possess chromatophores of several colours (blue, purple, pink brown and black). These colour changes can be very rapid.

The octopuses (octopi, if you want to get "picky"!!) are the masters of these rapid colour changes. Changes are usually initiated by visual input. In the octopus, the chromatophores consists of three sacs containing different colours. These are activated and the colours are adjusted until the octopus is camouflaged. Colouration, scientists now believe, also indicates an octopus mood. Normally brown in colour, when they turn white they may be showing fear and when they turn red it may indicate that they are angry (just like people!!). This gives the cephalopods the ability to communicate by using colour changes.

Cephalopods are all carnivorous. A radula is present, but even more important is the pair of powerful beak-like jaws that enable the animal to bite and tear apart their food.

The squid prey on fish and various invertebrates. When going after fish, squid dash in, grab a fish, pull it in and bite a triangular chunk out of its neck, thus severing its nerve cord. Squid possess ten arms arranged in five pair around their head. The eight short heavy appendages are called arms. The other two, which are twice the length, are called tentacles. The inner surface of each arm is flattened and covered with stalked, cup-shaped adhesive discs that function as suction cups. Often hooks are also located around the periphery of each disc. Attached to the floor of each suction disk are muscle fibers. When a squid touches its prey with these suction cups, the muscle fibers contract and cause suction. The two long tentacles are very mobile and not flattened until their end. The spatulate ends bear suction discs and are used for grasping prey and drawing it to their mouths.

The octopi dwell on the ocean floor and they usually just wait for their favorite meal of shrimps, crabs fish and other molluscs to venture within their reach. Octopods have eight tentacles, of equal length, covered with sessile suckers. They do not possess a horny ring or hooks on their suckers, as do the squid. The octopus wraps his tentacles around its prey and with suckers firmly attached, he pulls the prey to his parrot-like beak (mouth), where it is immobilized (see below), then torn apart and devoured. Many octopi are active, intelligent hunters and scavengers that will eat nearly anything, and are VERY quick!! This is why they tend to end up as the only inhabitants of fish tanks they are accidentally or innocently deposited in.

The nautilids possess approximately 94 tentacles that arise from lobes that are arranged in both the inner and outer circle around their head. These tentacles lack sucker or adhesive discs. They are instead annulated and can be drawn into a sheath. Located above the head and tentacles is a leathery hood which can be pulled over the vulnerable soft body parts, which the animal pulls into its shell when, threatened.

The beak is located in the buccal cavity and is used to bite and tear off large pieces of tissue. These morsels are then shredded and drawn into the buccal cavity by means of the radula (think of the chain on a chainsaw going around). The squid and octopus possess two pair of salivary glands that empty into the buccal cavity. The one pair of glands secretes a digestive enzyme while the second pair produces a toxin. This poison enters the prey through the wound inflicted by the beak bite.

The stomach is very muscular and a large cecum is attached at the anterior. In Loligo the cecum is straight. In Sepia, Octopus, and Nautilus it is spiraled. The digestive glad in cephalopods is divided into a small, semi-distinct portion, sometimes referred to as a pancreas. This pancreas constantly produces a digestive enzyme, which is stored in the cecum. They also possess a large liver which is roughly homologous (i.e., it serves similar functions) to the digestive gland in the other molluscs. Hepatic secretions only enter the stomach during feeding.

When food enters the stomach, both pancreatic and hepatic secretions are poured into the stomach via a groove in the cecal wall. The food is churned and mixed with these digestive enzymes - this is how digestion begins, in the stomach. This partly digested slurry is then passed into the cecum where it is completed. The anterior (front) walls of the cecum contain an elaborate series of spiral ciliated folds, which separate nondigestible particles from the digestible cecal contents, which are retained, while the indigestible material is passed on to the intestine. Absorption takes place entirely in the cecum and the residue left over is also passed into the intestine. The straight or coiled intestine then carries this waste to the anus, which opens into the mantle cavity.

The two nephridia characteristic of both the squid and octopods follow the basic molluscan design. They are, however, rather compact and sac-like. The reno-pericardial canal (blood vessel from the heart (cardia) to the kidneys (the "reno" part)) has become enclosed within this sac. The pancreas is also embedded in the sac and plays a role in waste removal. Through the middle of each kidney passes a vein surrounded by glandular tissue. This tissue picks up the waste from the blood and deposits it into the sac cavity. The nephridiopores open into the mantle cavity near the anus. The siphon, which is attached to the underside of the head, extends into the mantle cavity and through this funnel body's wastes are expelled. (The cephalopod's eggs and ink are also expelled through this funnel)

The nephridia of the nautilus are somewhat different. They possess four nephridia and they have lost all connection with the pericardial cavity.

Cephalopods usually are usually dioecious (two sexes), and fertilization is internal. Courtship, which in some species can be quite elaborate, is often a precursor to copulation. Gonads are located in the posterior of the body.

In some species, males can be distinguished by the modified sucker discs found at the tips of his longer two tentacles. The male uses these long arms to remove a sperm packet from his mantle cavity and to then insert this packet into the female s mantle cavity. This arm is autonomous and sometimes can get broken off while mating and is retained within the female s cavity. If this happens, the male simply grows a new one: no problem!!

Within two months of mating, the female octopus will attach long strands of clustered eggs (resembling a cluster of grapes) to the ceiling of her lair. While the eggs are incubating, the female will gently caress them to keep them clean and free of bacteria. She also keeps a steady flow of fresh, oxygenated water flowing over her precious eggs. When they are ready to hatch, her caresses become more rigorous, which helps the young to escape from their egg sacs.

The pelagic Cephalopod family Argonautidae, commonly know as the paper nautilus, have a remarkable adaptation for egg deposition. The two dorsal arms of the female are greatly expanded at their tip to form a membrane. The expanded portion of each arm secretes one half of a beautiful calcareous bivalved shell. She then deposits her eggs directly into this case. The shell acts as both a brood chamber and a retreat for the female. The posterior of the female usually remains in this shell. The male, which is much smaller than the female, does not have such a shell and is often found in cohabitation within the same shell, more or less as a freeloader! (Photo)

Please Note: These are the actual web addresses that I used for researching the material found in this article. No attempt will be made to keep them current. To find up-to-date web pages, please visit my Internet Resources Section.

If a web page ceases to exist, I will leave the original URL in scrip; however it will not be linked..

AAA721 Invertebrate Zoology:
(http://www.bhtafe.edu.au/scitech/Invert.htm)
These resources were produced by Anne Jones, Box Hill Institute of TAFE, in 1994. They were used to support the delivery of the old Victorian Invertebrate Zoology subject. It does not have the same focus as the new national module; but is nonetheless useful reference material. In particular the pracs can be used.

Appendix D. Glossary of Molluscan Terms:
http://www.nature.nps.gov/wrd/mussels/mussels5-13.htm

Bivalve Anatomy:
http://www.assateague.com/nt-bival.html
Check out Seashells and Other Things from Assateague Beach homepage for some very interesting reading. Excellent cephalopod paper and pictures. Their home page also leads to many more very good web pages on the other molluscs Very. informative and some excellent diagrams to be found here.
http://www.assateague.com/shells.html

The Cephalopod Page:
http://is.dal.ca/~ceph/TCP/index.html
Check out this very informative site designed and edited by James B. Wood on Cephalopods. It is very well done and fun as well.

Cephalopoda Cuvier, 1797:
http://www.soest.hawaii.edu/tree/cephalopoda/cephalopoda.html
An excellent cephalopod site by Richard E. Young, Michael Vecchione, and Katharina M. Mangold * An excellent cephalopod paper and pictures. Their home page also leads to many more very good web pages on the other molluscs

Chp 26: Invertebrates:
(ttp://biology.nwc.whecn.edu/biology/biol2020/INVERTS.HTM)
Textbook study of invertebrates
I. What is an animal?
A. Most known organisms are animals (kingdom Animalia)
1. Of the 1.5 million known species, one-third are animals.
2. Most animal species live on land, but the greatest diversity of animal phyla are marine. The most diverse communities of animals are tropical coral reefs; 27 of the 30 animal phyla may be found here.

Class Gastropoda:
http://animaldiversity.ummz.umich.edu/mollusca/gastropoda.html
University of Michigan- Museum of Zoology - Animal Diversity Web. Check into this site and their home page for other excellent molluscs articles.

Class Gastropoda:
(http://www.geocities.com/coneshells/pages/e_pages/class_gastropoda.htm)

Classification of the Phylum Molluscs:
http://orion1.paisley.ac.uk/courses/Tatner/biomedia/units/moll1.htm
A very informative introduction to the molluscs at a university entrance level. There are even four test question sections that are fun to do.

Did you know an octopus has three hearts?
http://www.mote.org/~lauren/motenews/winter96/octopus.phtml
An informative site on the cephalopod and some other very interesting links such as the 1999 Giant Squid Expedition

Featured Creatures:
(http://www.ifas/ufl.edu/~insect/misc/florida_slugs.htm)

Giancarlo Paganelli's Cone Shells:
http://www.coneshell.net/ Good site and lots of links to check out.

Glossary of Molluscan Terminology:
(http://www.mbl.edu/html/KEYS/INVERTS/14/glossary.html)

Invertebrate Zoology Course: This site contains good university-level information on all the invertebrates - including molluscs. There are references to a textbook, but a surprising amount can be gleaned from these notes - For those keeners who want to DIG DEEPER!!
http://zoology.okstate.edu/zoo_lrc/zool3104/zool3104.htm

Man and Mollusc Internet Resources Section:
/links_index.html
Many more Malacological, conchological and other links.

Mollusca:
(http://biodidac.bio.uottawa.ca/Thmbnails/BIVA009B-GIF.htm)
An excellent article and diagram of the bivalve stomach and crystalline
style and how it functions in digestion

The Molluscs:Part 3: the bivalves
http://www.aquarium-design.com/invert/bivalve.html
The bivalves are the second largest group of molluscs with about 8,000 described species. These organisms, as the name implies, have two shells. Be sure to check out Odyssea's Newsletter when visiting this site or go direct to:
http://www.masla.com/newscat.htm

Mollusk: Snails and other weird animals:
(http://www.oceanicresearch.org/mollusk.html )
Oceanic Research Group Dedicated to the conservation of the world's oceans through education. Check out this excellent resource site at (http://www.oceanicresearch.org/index.html )

Mollusks:
(http://kalypso.cybercom.net/~dhe/Meadow/mollusks.html )
Site apparently no longer available

Phylum Mollusca:
(http://wwwmc.nhmccd.edu/elc/learning_resource_center/mollusca.html)
Montgomery College Library and Learning Resource Center. Check out this site. A great source for molluscs of all kinds and some very good links.

Neopilina: A Living Fossil:
http://www.grisda.org/origins/03056.htm
On May 6, 1952, ten living specimens of an extraordinary mollusc were discovered. While trawling off the Pacific coast of Costa Rica, the Danish deep-sea "Galathea" expedition hauled these specimens to the ocean surface from a depth of 3590 meters. They were given the name Neopolina galathea and their discovery has been described as "the most dramatic one in the history of malacology." It was an unusual discovery in more than one way.

Nudibranchs:
http://www.diveoz.com.au/frame_set.asp
This site is dedicated to the study of Nudibranchs (Phylum: Mollusca Class: Gastropoda Subclass: Opisthobranchia, Order: Nudibranchia), the colourful and bizarre marine slugs found throughout the world's seas and oceans.

Phylum Molluscs:
I was unable to get into this site directly but you can go through another door here: Welcome to Northern Michigan University's WEB http://vm.nmu.edu/
It's a good schematic paper of the Phylum molluscs

Reefkeepers's Guide to Invertebrate Zoology:
http://www.aquarium.net/flash_index.html
There are some great articles and lecture notes by Rob Toonen on molluscs in this site. There are quite a few excellent articles on their Aquarium Net as well and also notes on joining their Reef-List.

Shell:
(http://www.comptons.com/ceo99-cgi/article?'fastweb?getdoc+viewcomptons+AP+20264+0++Mollusks' )
Compton's Encyclopedia is a good source for finding basic information on molluscs and their shells and for related subjects such as pearls.

Snails Not Slow at Evolution:
http://www.abcnews.go.com/sections/science/DyeHard/dye990616.html
Have fun checking out the many sites available on this ABC News Science page. This is just one of many of their articles that I have read. Good site for kids and adults alike.

The following three sites are private sites and require permission from the owner to view them. They are excellent papers and worth pursuing the University of Warwick, Biological Sciences:

World Sea Shells:
(http://192.150.195.217/docs/shells.htm )
An excellent site for all sorts of pictures of molluscs and some basic information.

Invertebrate Zoology 6th edition
Robert D Barnes, PhD and E.E. Ruppert
Published by W. B. Saunders Co. 1994

Invertebrates
Kozloff. E.N.
Saunders Publishing, Harcourt
Brace College Publishers, Philadelphia. PA 1990

Biology of the Invertebrates
Jan A Pechenik
Published by Prindle, Weber & Schmidt, Boston 1985

College Zoology
Hegner and Stiles
Published by the Macmillan Company, New York 1963

Reader's Digest Illustrated Encyclopedic Dictionary
Published by the Houghton Mifflin Company, Boston 1987

The Velliger A Glossary of A Thousand-and-One Terms used in Conchology
Compiled by Winifred H. Arnold March 15, 1965

All the World's Animals Aquatic Invertebrates
Torstar Books
New York, NY 10017 1985

1. Subclass Prosobranchia: (proso-branch-i-a) Latin: pros=front branch=gill The majority of the gastropods are prosobranchs. This group includes all the gastropods that respire by means of gills and in which the mantle cavity, gill and anus are located at the anterior of the body. They possess a shell and torsion is evident. Most prosobranchs are aquatic. Many have also developed a stone-like (calcareous, made from calcium carbonate, like the shell) or horny (made of chitinous material similar to our fingernails, only not quite as hard) operculum. (For details on this operculum, read the foot and locomotion section) Some members of this subclass are the limpets, periwinkles, conchs, whelks, cones, murexes, cowries and volutes. Oddly enough, the most successful family of all the molluscs, the Turridae, which are among the most advanced Prosobranchs, are little known - they are often quite small, and never found along the seashore. They have nearly 7,000 described species so far.
		Order Archeogastropoda (or Aspidobranchia ): (Arch-e-o-gas-tro-poda) Latin: arch=ancient eo=dawn gastro=stomach poda=foot Primitive prosobranchs in which there are two auricles, two kidneys and two gills present. Nerve system is never concentrated. Shell is either coiled or secondarily symmetrical as in the limpets. Largely marine but there are a few that inhabit brackish water, freshwater or even terrestrial habitats. Examples: Pleurotomariidae (Slit shells), Halitidae (Abalone), and Trochidae (the Top shells). Most have operculi, and all are marine inhabitants.
		Order Mesogastropoda ( or “Pectinibranchia”): (Me-so-gas-tro-poda) Latin: : meso=middle gastro=stomac poda=foot Mesogastropods possess one gill, one auricle and one kidney. An operculum may be present. Mostly marine but a few do inhabit freshwater. This is the largest order of gastropods and contains many common species, such as the Littorina, Janthina, Crepidula, Stromb, Lambis, Cerithium, Polinices, Vermetidae worm-shelled snails, Pomatiasidae and Cyclophoridae.
		Order Neogastropoda (Stenoglossa): (Ne-o-gas-tro-poda) Latin: neo=new gastro=stomach poda=foot Neogastropods possess a concentrated nervous system and usually a shell with a siphonal canal. They are a carnivorous species having a radula containing two or three large teeth in each row. Some possess a poison gland. Nearly all have an operculum. All are marine inhabitants. This group includes the beautiful Murex, Busycon, Colus, Fasciolaria, Conus, Turridae and Terebra.

2. Subclass Opisthobranchia: (o-pis-tho-branch-ia) Latin: opistho=behind branch=gill The opisthobranchs display various stages of detorsion. Many have adapted a secondary bilateral (i.e., two-sided, as in humans!) symmetry in which the shell is either much reduced or completely absent. Gills are generally posterior (i.e., behind) to the heart and are often on the outside of their bodies in the form of plumes. They possess one auricle (heart chamber) and one kidney. They are all marine inhabitants, and many have adapted to a pelagic or swimming style of life. Most are herbivorous, but many are parasitic (e.g. pyramidellas), living on other bivalves and sea creatures. Some of the more familiar opisthobranchs animals are; those with diminished shells, the sea hares and those with no shell at all, the nudibranchs and sea slugs. A few such as the bubble shells do possess a hard shell.
	Order Tectibranchia: Latin: tect=covered branch=gill Shell is present; however it is often much reduced or covered by the mantle. They possess one true gill. Many are secondarily symmetrical. All are marine inhabitants. Some members are the Acteon, Bulla, Scaphander, Philine, Aplysia, Pleurobranchus, and the Pyrums.
	Order Pteropoda: Latin: pter=wing pod=foot Here the anterior portion of the foot has expanded to form swimming fins. These sea butterflies (As opposed to the sea hares , which are different again!) may or may not have a shell. The Pteropods are marine inhabitants and we know such members as Spiratella, Clio, Cavolina, Limacina, and Cuvierina. Their shells are most often found in fine, deep-sea sediments, where they are not lost or crushed amongst coarser sand, gravel and rocks.
	Order Nudibranchia: Latin: nud=naked branch=gill These are the shell-less sea slugs ornudibranchs. They are secondarily symmetrical (whatever that means!). They do not possess a mantle cavity or gill. Respiration is through the body surface, cerata or secondary gills located around the anus. Their nervous system is concentrated. All are marine inhabitants and we know them as the Doris, Dendronotus, Elysia, and Aeolidia. As a bonus for us humans, they are amongst the most beautiful creatures in the ocean - take a browse through the Nudibranch and Sea Slug Sites found in the links section, and I GUARANTEE you'll be surprised!

3. Subclass Pulmonata: (pul-mon-a-ta) Latin: pulmo=lung (because they breathe air) The pulmonates retained the post torsion anterior position of the anus and mantle cavity; however, the gills have disappeared and the mantle cavity has become modified into a lung . They possess one auricle and one kidney. This subclass contains most of the woodland and garden snails. Garden slugs are pulmonate snails that have evolved without developing a shell, or that have perhaps lost them somewhere in the mists of time. Many freshwater snails are also pulmonates.
	Order Stylommatophora: Latin: styl=column omm=eye phor=carry eyes carried at the end of stalks (tentacle) Stylommatophors possess two pair of tentacles with eyes located at the tip of the posterior pair. All are terrestrial. Included are the Helix (the family of the famous French Escargot!), Polygyra, Pupa, Janella, Deroceras, Philomycus, Palifer, Testacella and Limax.
	Order Basommatophora: Latin: basi=bottom omm=eyes phor=carry : eyes carried at the base of stalk (tentacle) Basommatophors possess one pair of tentacles with eyes located near the tentacle base. They are primarily freshwater inhabitants and they require air for respiration although some do take water into their mantle cavity and have evolved secondary gills. Some are terrestrial inhabitants and a few are marine inhabitants. These include such familiar families as the Phasionellidae, and many are left-handed (coiling to the left, instead of to the right, like most marine species do). Most have thin, fragile shells, since they don't have to put up with the rough-and-tumble of the waves in the ocean.

The Pelecypoda consists of three Subclasses, which are further divided into superorders, orders then almost 120 families, and over 15,000 known species:

Subclass: 1. Protobranchia (Pro-so-branch-ia) Latin: proto=front branch=gill: Primitive bivalves, their gills are not folded. Palpal proboscides are frequently present. Included are the Nuculanidae (Nucula, Nuculana, Yolida, etc.). Solemyidae, and Malletidae - not commonly-known families, since they are not very colorful, and most are rather small.
	Order: Nuculoida (= Paleotaxodonta): Shell is aragonic with an interior that is nacerous or porcelaneous The periostracum is smooth. The valves are equal and have a row of sharp teeth along its hinge or border. Large palps used for food collection. Ctenidia are small and used only for gas exchange. Foot is longitudinally grooved and has a plantar sole.
	Order: Solemyoida (=Cryptodonta): Shell valves are thin, equal in size, elongate and lacking hinge teeth. They have a large ctenidae used for both feeding and gas exchange. Their palps are small.

Subclass: 2. Lamellibranchia: (La-mel-li-branch-i-a) Latin: Lamella=leaf; branch=gill: Shell valves are quite thin, elongated and equal in size.. The valves are uncalcified along the outer edges and hinge teeth are absent. They have one large ctenidia which is used both for feeding and gas exchange. Their palps are small.
	Superorder: Filibranchia (Phil-I-branch-ia) Latin: fil=thread branc=gill (also known as Pteriomorphia): They possess attenuated, flexed gill filaments. The filaments are incompletely fused; intercellular junctions are present but the adjacent filaments are joined only by ciliary tufts. They are primitive bivalves and include the mussels-Mytilidae (Mytilus and Modiolus are the most famous genera); the ark shells (Arcidae); the oysters (Ostreidae (Ostraea and Crassostrea being the most well-known families), Spondylidae - the spiny oysters ; Anomia (the jingle shells ); Limidae, or Lima - bean shells; the very famous Scallop family (Pectinidae - the Pectens); and the wood - boring Lithophaga (shipworms).
	Superorder: Eulamellibranchia: (Eu-la-melli-branch-ia) Latin: eu=well, very lamella=leaf, layer branch=gill (also called the Paleoheterodonta and the Heterodonta - two subclasses often just lumped together as the Eulamellibranchs (and no, I don't know why nearly every taxonomic group in the bivalves has at least two names!!) The reflexed gill filaments are morphologically fused to form true lamellae. Includes the families Lasaeidae; Cardiidae (the cockle shells ; the common edible clam-Mercenariidae (Quahogs, in New England and further south); the boring clams-, Hiatellidae; Martesiidae, and Toredinae (as in the genus Toredo), which are another family of shipworms (they are 4 families all-told); the razor clams ; The Donax or wedge clam family; Pholadidae (the Angel Wings ); Mactridae; and the freshwater families Unionidae, Sphaeriidae and Margaritiferidae, and the familiar Veneridae - the Venus Clams, and Tridacnidae - the Giant Clams!
		Order: Paleoheterodonta: There are about 1,200 species and it includes the nearly extinct family Trigoniidae (fewer than 6 living species) and the Unionoidea (fresh water bivalves)
		Order: Veneroida: Usually thick-valved, equal valved and isomyarian. Includes the families: Cardiidae, Tridacnidae, Solenidae, Tellinidae, Semelids, Donacidae, Veneridae, Sphaeriidae (freshwater) and Corbiculidae (freshwater) .
		Order: Myoida: Thin-shelled, burrowing bivalves with well developed syphons. Includes the families: Pholadidae, Tereinidae and Corbulidae

Subclass: Anomalodesmata: (A-nom-o-des-mat-a) Contains the Septibranchia (Sept-a-branch-ia) Latin: sept=seven, wall branch=gill (also called Anomalodesmata - and no, that's not going to be on the test: --) ): a small, specialized group, in which gills are not present. The inhalent and suprabranchial (exhalent) cavity are separated by a pumping septum. Includes 13 families you probably never heard of, such as Poromyidae and Cuspidariidae.
		Order: Pholadomyoida

Note:
Glossary and separate page links are in blue and underlined. You may have to use your back button to navigate back to article when in the glossary or in external web sites or pages.

Chaetoderma	Neomenia