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Molluscs are a large and diverse group of animals. Familiar molluscs include snails, clams, squids, and octopuses. Phylum Mollusca (molluscs): Animals with a true coelom of the schizocoel type, usually bearing a shell composed mostly of calcium carbonate and secreted by a mantle. The mantle is always withdrawn at the rear to form a mantle cavity, which contains anus and gills. Primitive molluscs and gastropods use a tongue-like radula with embedded teeth to scrape encrusted algae from rock surfaces. Class Monoplacophora (primitive molluscs): Molluscs with a simple dome-shaped or low conical shell. Muscles, blood vessels, and other structures segmentally arranged. Digestive tract simple. Class Gastropoda (snails and slugs): Body usually undergoes asymmetrical torsion (twisting and coiling). One-piece (univalve) shell, usually coiled. Most species herbivorous. Well-developed head, sense organs, and nervous system. Locomotion typically by creeping on a muscular foot. Class Polyplacophora (chitons): Simple, flattened body, with shell divided into several overlapping plates that permit some flexibility. Head small but radula well-developed and used in feeding. Class Bivalvia or Pelecypoda (clams and other bivalves): Body usually symmetrical, narrowly compressed from side to side. Two-piece (bivalve) shell; left and right halves are often mirror images (except at hinge). Many species filter-feed, straining small particles from the water. Head and sense organs poorly developed. Muscular foot hatchet-shaped (flattened side-to-side), often used in burrowing. Class Scaphopoda (tusk-shells): Small molluscs with tusk-like shells. Mantle cavity runs for entire length of shell along posterior margin; water passes through mantle cavity, exiting through hole at the top. Class Cephalopoda (octopuses, squids, nautiloids, etc.): Body usually symmetrical. One-piece shell is symmetrically curved or coiled in median plane, or often lost. Most species are actively swimming predators. Head very well-developed, with sense organs (especially eyes), brain, and beak. Muscular foot subdivided into numerous tentacles. Body doubled over, with mantle cavity (originally rear) tucked beneath head and opening forward. Frequent "ink glands" that secrete dark, inky fluid to confuse predators. |
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