Phylum MOLLUSCA

Distantly related to annelid (segmented) worms and arthropods, this extremely diverse phylum has been extraordinarily successful.  Mollusks make up 11% of all animal species that have ever lived.  Man himself has made steady use of this group as a significant source of protein.  Some molluscs live in trees while others live up to 35,000 feet below sea level.   Most members of the phylum have external shells and they all show a tendency to coil to a greater or lesser extent.  There are eights molluscan classes of which only three are important in the fossil record: the Gastropoda, Pelecypoda (=Bivalvia) and Cephalopoda.

Class GASTROPODA (Snails and slugs)

When present (it is absent in slugs for instance), the shell is single, closed at the point (in contrast to the Scaphopods), often spiral and lacking chambers (in contrast to Cephalopods which may have similarly spiraled shells and have chambers).  Generally, the shell is not planispirally (flat) coiled but is conically coiled.  Gastropods are chiefly herbivores.  Some though, are carnivores and their carnivorous feeding adaptation include boring into other molluscs.  Among more bizarre ones, the slipper limpet Crepidula simply settles over oysters and smothers them before dining on them. More pictures of Gastropods

Class CEPHALOPODA (Nautiloids Ammonites Cuttlefish and Squids)

Cephalopod shells generally resemble gastropod shells.  Although planispiral (flat) coiling predominates, it is not absolute. Conical, as well as straight forms, are also present.  The shell is chambered and the animal lives only in the last chamber (in gastropods the animal occupies virtually the entire shell).  Some groups (e.g. cuttlefishes and belemnites) have lost their outer shell and have developed internal skeleta while others (squids) have completely lost their skeleton.  The following taxa of Cephalopods have been particularly important in the fossil record: Cephalopod Subclasses

Class PELECYPODA (= Bivalvia) Clams

The two valved shell which gives the class its name is hinged.  Generally speaking, members of this class are symmetrical along the hinge line but not bilaterally symmetrical (as are brachiopods).  However, some common clam groups, in particular the oysters, do not follow this arrangement, are often asymmetrical and therefore cannot be recognized on the basis of symmetry.