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The calcareous spicules that cover the bodies of most aplacophorans give the animals a
striking sheen. These spicules are secreted by the mantle epidermis and are the probable
homologue of the shell of other molluscan groups. Spicule morphology varies over the body
of the aplacophoran, and in some taxa spicules are modified into scales.
Overall, the aplacophoran body plan is similar to that of the chitons. Aplacophorans and polyplacophorans differ from the monoplacophorans by having a dorsal gonad rather than a posterior gonad. The pericardium is similar in all three groups as are many of the other organ systems and positions. Major differences are found in the type of spicules secreted by the dorsal mantle epidermis. |
The internal anatomy provides evidence of the molluscan identity of aplacophorans.
In both groups the anterior end of the alimentary system includes a radula and odontophore. In
the Chaetodermomorpha the radula and odontophore are
strongly developed, and the alimentary
system is more differentiated than in the Neomeniomorpha.
Both groups have a dorsal gonad that
opens into the pericardium, which contains the heart. From the posterior portion of the pericardium
a duct (actually several coelomoducts)extends out and loops around before ultimately
opening into the pallial cavity.
The nervous system is simple and ladder-like with a
well-developed cerebral ganglion.
Radular configurations are quite variable and show a wide range of tooth development and modifications that include jaw-like structures, denticles with cones, and sweepers. This is second only to the range of radular variation found in gastropods and is in marked contrast to the lack of variation found in Monoplacophora, Polyplacophora and Scaphopoda. |
