The Mollusca is one of the most successful phyla in terms of species richness and abundance with over 100,000 living species. Members of this phylum are very diverse with a range of forms including bivalves, gastropods, squid and octopus. Many molluscs are externally shelled, but some such as nudibranchs and aplaophorans are not.
Bivalves are distinguishable by two approximately symmetrical shells connected by a hinged region of calcified teeth and a tough ligament which springs the shell open. Large muscles counter this force and close the shell. Many soft-bottom dwelling bivalves have a large foot which can be used to move the shell, and some such as scallops can swim by forcing water through the siphon and others such as mussels attach to hard substrate using secreted material called byssal threads. Gastropods have a flattened foot muscle, topped by a shell with a single opening and an organic door known as an operculum. Most gastropods are detrital feeders but some can be carnivorous. Cephalopoda (squid and octopus) are complex molluscs with elaborate nervous and muscular coordination, high visual acuity, suckered arms and carnivorous feeding habits.