| Class Polyplacophora |
||||
 |
||||
| The chitons (ky-tons) in Class Polyplacophora (many shell bearer) show most of the molluscan characteristics in essentially the ancestral condition. The most obvious exception is the shell, which is divided into eight individual valves or plates. These plates are attached to one another via stout ligaments and muscles yet articulate freely enough to allow the chiton to conform to the irregularities of their rocky substrate habitats. Indeed many of the chitons can roll up, pill-bug style, if they are detached from the rocks, a behavior that gives them their other common name, the sea cradles. The chiton creeps along on its broad muscular foot, much like the ancestral mollusc. When at rest the foot can obtain a hold on the substratum that is quite surprising. Observe a chiton in the laboratory. If it is attached to the aquarium wall you can see the size of the foot. The mantle of the chiton is quite thickened on its border to enclose the edges of the shell plates and overhang the foot. The thickened mantle is called the girdle and it may be adorned with bristles or hairs. You may be able to see the ctenidia (gills) that occur in the mantle cavity that extends on each side of the chiton between the foot and the girdle. Locate the rounded head region with the centrally located mouth. The chiton has a large radula that it uses to rasp algae off the rocky substrata. The individual radula teeth are coated with a metallic layer, called magnetite, that strengthen them considerably.
There are approximately 600 species of chitons, occurring worldwide in marine habitats. They are usually found in shallow, rocky environments where their algal food grows in profusion and their strong foot allows them to hold on against the force of the waves. There are over 100 chiton species along the west coast of North America, one of the richer chiton faunas in the world. |
||||