Phylum Mollusca

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The molluscs rival the arthropods in their diversity of body forms and sizes, as well as their ecological success. The phylum also provides some of the most familiar animals, such as snails , clams , mussels , squids , and octopus. The phylum Mollusca also includes lesser known forms such as the chitons , tusk shells, solenogasters , among others. Approximately 50,000 species of Molluscs have been described, and because of the shelled forms they have left a rich fossil record. However, the earliest molluscs probably arose in the Precambrian , and because early forms without shells left little in the way of fossils, nothing is currently known about what they were like. Systematic summary for the phylum Mollusca : Class Gastropoda ( snails, sea slugs) Class Pelecypoda (=Bivalvia: clams, mussels, oysters, scallops) Class Cephalopoda (squids, octopus, nautilus, cuttlefish) Class Aplacophora Class Monoplacophora Class Polyplacophora (chitons) Class Scaphopoda (tusk shells) Class Caudofoveata CLASSES OF MOLLUSCS Class Gastropoda The Class gastropoda consists of forms that are strongly cephalized (well developed head), with bodies typically coiled or torted (many forms later undergo detorsion during development which straightens the body out). The body is also dorsoventrally elongated. Various Nudibranch (sea slug) species, class Gastropoda , Order Nudibranchia , from Johnson and Snook, 1927. Most of the approximately 40,000 living species of gastropods have shells, however there are quite a few groups that have either reduced or internal shells, or no shell at all. Shelled forms are generally called "snails " and forms without shells are called "slugs", however the terrestrial slugs are not closely related to the various marine forms that are without shells. Although most Gastropods are marine, there are numerous forms in both freshwater and terrestrial environments. Class Pelecypoda (=Bivalvia ) Scallop , Pecten sp., class Pelecypoda Modified from Furneaux, 1908 The bivalves include forms such as clams , mussels , scallops , oysters , as well as many less familiar forms. Bivalves are laterally compressed animals, with two shell "valves " that are hinged on the animal's dorsal surface. The approximately 8,000 living species of bivalves are mostly marine, but there are many freshwater species as well (however, no terrestrial ones). Bivalves are found in just about every marine environment, from the intertidal zone to the deepest marine habitats. Most bivalves are suspension feeders, filtering small organisms and organic particles from the water (such as bacteria , phytoplankton , zooplankton , and nonliving organic detritus ). Class Cephalopoda Squid , Ommastrephes sp., class Cephalopoda , from Halstead , 1965. Cephalopods , which include the familiar squids and octopus , include species which are the largest known invertebrates (giant squid , up to 20 meters long, including tentacles), the most intelligent, and the fastest swimming aquatic invertebrates (squid ). There are also forms with external shells (nautilus ), and internal hard shells (cuttlefish ). Most of the approximately 650 living species of Cephalopods are active swimmers, however most species of octopus have secondarily assumed a benthic existence. Cephalopods have a closed circulatory system, an adaptation to their active lifestyle, as opposed to the open circulatory system found in other molluscs . Squid in particular are often very abundant in pelagic marine environments, where they are voracious predators of many organisms, especially fish. In return, they are also the major prey item of many fish and some marine mammal species. Class Aplacophora The class Aplacophora (animals also known as Solenogasters ) consists mostly of small worm-like molluscs that live symbiotically (or feed upon) cnidarians . They have no shell, eyes, or tentacles. There are about 250 described species, usually found in deeper waters (over 200 meters depth), where they are sometimes quite abundant. Class (or subclass) Caudofoveata There are only about 70 described living species of this class, which are small, worm-like molluscs that live buried head down in the sea floor. Relatively little is known about them. Many biologists consider this a subclass of the Aplacophora, which they resemble. Class Monoplacophora Monoplacophoran , ventral view, from BIODIDAC , Univ. of Ottowa Living members of the class Monoplacophora were first discovered in 1952, and presently the class consists of only 11 described living species, all of which live in deep sea environments. Monoplacophorans have a single cap-like shell, so they superficially resemble limpets . However, they possess many primitive features, and studies of their internal anatomy (they were previously known only from fossils ) have provided much fuel for debates about molluscan evolution. Class Polyplacophora Chiton , Nuttalina californica , class Polyplacophora , from Pratt, 1923. The Polyplacophora , commonly known as chitons , have seven or eight dorsal shell plates (although they may be covered mostly or entirely by soft tissue in some species). The approximately 600 described species are generally flattened and elongated animals that are typically found in the intertidal zone grazing on epibenthic algae . However, a few deep sea species have been described. Class Scaphopoda Tusk shell, Dentalium sp., class Scaphopoda , from Pratt, 1923. The Scaphopoda (tusk shells) have a tapered, tubular shell that is open at both ends, superficially resembling the tusk of some mammals . The approximately 350 described species are all marine forms, usually between 3 and 6 cm. long (the maximum size of a living species is 15 cm.). Scaphopods lie buried in soft sediments (usually sand), in shallow to moderately deep water (usually less than 2,000 meters deep), the larger end of the shell (containing the head and foot) facing downward, and with the smaller aperture projecting above the surface. The mouth area is surrounded by tentacles bearing adhesive knobs, which capture small organisms such as foraminiferans . For more information beyond this site, search the web! Suggestions for improving, changing, or adding to this website and etextbook should be emailed to:

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