(Alternate name: Loricata)
Latin meaning: poly = many plac = plate phor = carry, i.e.: bearer of many plates.
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!)
|Shell & Mantle: (Diagram)|
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.
|Foot and Locomotion: (Diagram)|
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.)
|Water Circulation and Respiration: (Diagram)|
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.
|Nervous System and Sensory Organs: (Diagram)|
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.
|Nutrition & Digestive System: (Diagram)|
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.