Gastropoda: Morphology

Gastropods are characterised by the possession of a single (often coiled) shell, although this is lost in some slug groups, and a body that has undergone torsion so that the pallial cavity faces forwards. They have a well-developed head that bears eyes and a pair of tentacles (cephalic tentacles) and a muscular foot used for "creeping" in most species while in some it is modified for swimming or burrowing. The foot typically bears an operculum that seals the shell opening (aperture) when the head-foot is retracted into the shell. While this structure is present in all gastropod veliger larvae, it is absent in the embryos of some direct developing taxa and in the juveniles and adults of many heterobranchs. The nervous and circulatory systems are well developed with the concentration of nerve ganglia being a common evolutionary theme.

The shell is typically coiled, usually dextrally, the axis of coiling being around a central columella to which a large retractor muscle is attached. The opening of the shell (aperture), into which the animal can typically retract, is often sealed with a horny (sometimes calcareous) operculum. The uppermost part of the shell is formed from the larval shell (the protoconch). The shell is partly or entirely lost in the juveniles or adults of some groups - with total loss occurring in several groups of land slugs and sea slugs (nudibranchs).

Externally, gastropods appear to be bilaterally symmetrical. However, they are one of the most successful clades of asymmetric organisms known. The ancestral state of this group is clearly bilateral symmetry (e.g., chitons, cephalopods, bivalves, see above), but gastropod molluscs twist their organ systems into figure-eights, differentially develop or lose organs on either side of their midline, and generate shells that coil to the right or left. The best documented source of gastropod asymmetry is the developmental process known as torsion.

Externally the animal has a well-developed head bearing a pair of cephalic tentacles and eyes that are primitively situated near the outer bases of the tentacles. In some taxa the eyes are located on short to long eye stalkseyestalks. The mantle edge in some taxa is extended anteriorly to form an inhalant siphon and this is sometimes associated with an elongation of the aperture of the shell. The foot is usually rather large and is typically used for crawling. It can be modified for burrowing, leaping (as in conchs - Strombidae), swimming or clamping (as in limpets).

A detailed exhibit of gastropod anatomy has been prepared by Andrew Zoltan Mason at California State University - Long Beach.