Phylum: Mollusca

NOAA
An octopus living near a seafloor volcanic vent.

Mollusks are a varied group of invertebrates, all with a muscular "foot", and many with some sort of shell (extruded from a mantle). Mollusks are put into three classes: Pelecypoda, gastropoda, and cephalopoda.

Class: Pelecypoda

Pelecypods are also called bivalves, because of their two-part shell. Some of the members of this class are clams, oysters, scallops, and mussels.

Vent Clam, found near under-sea volcanic vents. NOAA.

A giant clam. NOAA

Class: Gastropoda

Members of this class slide around on a slime trail exuded by their "foot". Many grow a shell from a mantle. Some of the members of this class are the slug, snail, and the nudibranch, a colorful sea slug.

National Aquarium in Baltimore

NOAA

Class: Cephalopoda

Cephalopoda means "head-foot". Members of this class have a prominent head/body structure, and a ring of tentacles surrounding a beak-like mouth. Most members have eight tentacles, while squid have two extra which are longer than the others, and the nautilus has up to ninety protruding from a snail-like shell! Some members of this class, like the squid and the octopus, are among the most intelligent of invertebrates. Cephalopods range in size from less than an inch to over fifty feet in length.

Nautilus. Gerald and Buff Corsi © California Academy of Sciences.

Squid. NOAA

Cuttlefish. John White.

The tiny and highly venomous Blue Ringed octopus. National Aquarium in Baltimore