MOLLUSCA - THE MOLLUSKS (CLAMS, SNAILS, CEPHALOPODS ET AL.)

The thorn shell, Acanthina angelica

The phylum Mollusca is the second most diverse phylum after Arthropoda with over 110,000 described species. Mollusks may be primitively segmented, but all but the monoplacophorans characteristically lack segmentation and have bodies that are to some degree spirally twisted (e.g. torsion). The Phylum Mollusca consist of 8 classes: 1) the Monoplacophora discovered in 1977; 2) the worm-like Aplacophora or solenogasters of the deep sea; 3) the also worm-like Caudofoveata; 4) the Polyplacophora, or chitons; 5) the Pelecypoda or bivalves; 6) the Gastropoda or snails; 7) the Scaphopoda, or tusk shells; and 8) the Cephalopoda that include among others squid and the octopus. On the left is a member of the very diverse Gastropoda. This particular example is the intertidal snail Acanthina angelica, which subsists by using the thorn like projection of the lip of its shell (arrow) to pry open the barnacle Chthalamus (see Arthropoda). Acanthina is a member of the Class Gastropoda, whose members are characterized by spiral shells, a fleshy foot, and a rasping organ called a radula. Other gastropods you are likely to see include the sea slugs and hares, limpets, conchs, murex, olive shells, conus, and many others.

The Pelecypoda are the bivalve mollusks and include oysters, clams, and scallops. The example to the right is the rock piddock, Pholas chiloensis. This bivalve burrows into rock, shells, or wood. It is responsible for many of the burrows you see in the beachrock. Members of the Phylum Pelecypoda often have a muscular foot which they use to push their way through a substrate. Although externally often symmetrical about a plane between the shells, internally they have no obvious symmetry.

The polyplacophoran, Chiton virgulatus and the sea slug (Class Gastropoda), Onchidella binneyi.

The Class Polyplacophora comprises the chitons, which are superficially bilaterally symmetrical and segmented. Internally, they show no segmentation, however. They are very common on rocks where they graze using a radula.

The Cephalopoda are named such because they have a prominent head that bears long flexible feet. The only cephalopods you are likely to see in the rocky intertidal zone are octopi. These highly intelligent mollusks have prominent eyes remarkably similar in structure to those of vertebrates. On the right is the relatively common Octopus bimaculatus, who's common name is, not surprisingly, the two spotted octopus. Octopi feed often on bivalves and crabs, the discarded shells of which frequently surround their lairs. Octopus bimaculatus is collected for food at Station Beach.

The two spotted octopus, Octopus bimaculatus.

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