Phylum Mollusca

Common Name: Snails and Slugs (Gastropods), Squids and Octopuses (Cephalopods), and Clams (Bivalves)

Etymology: "Soft-bodied", from the Latin molluscus meaning soft of body, probably in reference to the general lack of rigid structures in their bodies.

Representative examples:

Click images to enlarge.
Cryptochiton, the giant Pacific gumboot chiton	The cap-shaped shell of Neopilina	Perotrochus, a primitive gastropod (or “slit shell”)	The red abalone Haliotis refescens. Note the exhalant holes in the shell.	Epitonium scalare, the precious wentletrap

Additional Information:

The phylum Mollusca is a morphologically and ecologically diverse group of bilaterally symmetrical protostomes with true coeloms and complete digestive tracts. The molluscan body is unsegmented (distinguishing the group from annelids and arthropods) and has well-developed organ systems in comparison to more primitive, acoelomate or pseudocoelomate invertebrates like nematodes and flatworms. Although they come in a wide variety of forms and inhabit a diversity of environments, all molluscs share a basic body plan (diagrammed above right) consisting of a musucular foot (often involved in locomotion), a visceral mass containing internal organs, a fold of the body wall tissue called a mantle, and (often) an calcareous exoskeletal shell. These basic parts of the body are modified to fit the ecology of various subgroups within the Mollusca (see diagram below left). For example, the broad, flat, muscular foot of the snail is modified into the arms, tentacles, and siphon of the squid. Similarly, the thick shell of a bivalve mollusc like a clam is modified and reduced into the rigid internal pen found in seemingly shell-less cephalopods. We focus on three large groups of molluscs in lab: Class Bivalvia, which includes scallops, clams and mussels, Class Gastropoda, which includes snails, slugs, and sea snails like Busycon, the whelk, and Class Cephalopoda, which includes squids, octopuses, and the nautilus.

variations on the molluscan body plan

Links

• Mollusca (Earthlife.net)
• Octopus Arms Found to Have "Minds" of Their Own
• First images of giant squid in wild!

Click images to enlarge.
The bizarre anomalodesmatan clam Brechites.	Conus (a gastropod)	Chromodoris, a nudibranch	Monadenia fidelis, a terrestrial snail from California	Octopus

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Histioteuthis, a pelagic squid	Fustiaria, a tusk shell	Scallops, with a hermit crab in the foreground	Clinocardium, a cockle	Lima, a tropical clam that swims by clapping the valves together

Click images to enlarge.
Chlanys opercularis. A scallop	Calliostoma, a gastropod adapted to crawling on hard substrata. Note the line separating the right and left muscle masses in the rear of the foot; this allows a somewhat ‘bipedal-like’ motion as the animal moves	Sepia, the cuttlefish	Nautilus, a cephalopod	Dentalium vulgare. A scaphopod mollusk