CHITONS, BIVALVES, UNIVALVES, BUBBLE SHELLS, NUDIBRANCHS, TUSKSHELLS, OCTOPUS, and SQUID—Phylum MOLLUSCA (Molluska)

A superficial list includes chitons, mussels, scallops, oysters, clams, cockles, tellins, macomas, piddocks, shipworms, limpets, snails, abalones, dogwinkles, trophons, tritons, periwinkles, bittiums, turbans, olives, wentletraps, tusk shells, octopus, squid, bubble shells, sea hares and the various nudibranchs or sea slugs.
Each member of this large and conspicuous group has a soft, unsegmented body that includes a component called a mantle. This flap acts as a tent that envelops the remaining tissue—see photograph A, rough keyhole limpet (MC153) (upper) and spiny pink scallop (MC43). In the vast majority of species, for at least some portion of their life cycle, the mantle also constantly produces a calcareous secretion that hardens to form one or more shells. A typical snail shell is of hollow construction—see photograph B, wrinkled dogwinkle (MC167)—and provides space for the twisted body of the animal to grow continually. The muscular foot, another basic feature of every mollusc, may have evolved into one of many designs in response to a variety of functions.
Well over 1,600 species of molluscs live in the marine waters of the Pacific Northwest.

Further Reading:
Behrens, David W., and Alicia Hermosilla, 2005, Eastern Pacific Nudibranchs—A Guide to the Opisthobranchs from Alaska to Central America, Sea Challengers, Monterey, CA, 140 pp.
Coan, Eugene V., Paul Valentich Scott and Frank R. Bernard, 2000, Bivalve Seashells of Western North America, Santa Barbara Museum of Natural History, Stanford, CA, 764 pp.
Harbo, Rick M., 2001, Shells and Shellfish, Harbour Publishing, Madeira Park, BC, 271 pp.

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