GASTROPODS
Gastropods (a.k.a. "snails") are univalved molluscs (though some have opercula for the shell) with the body and (typically) the shell both "twisted". The twist in each case is not the same thing. The shell is twisted so as to coil trochospirally to allow shell expansion without requiring great elongation, while the body has undergone torsion so as to turn the digestive system and bring the anus to the aperture of the shell where it must be. This torsion is what gives the subphylum Cyrtosoma its name.
Some snails have no shell or an extremely reduced one, but those that do almost invariably have the trochospiral coiling pattern you learned about in Geology II. The photographs below show two examples: Terebra sp. from Sharm el Sheikh, Egypt on the left and Polinices duplicata from grand Isle, LA on the right.
Terebra, with its low apical angle, is a "high-spired" snail; Polinices, with a larger apical angle, is "low-spired". The distinction arises as a consequence of the very simple growth rules of the snails, a topic we will examine later in the term. The "apex" is the pointed, oldest end of the shell away from the "aperture", where the body emerges from the shell. The apex is at the "posterior" end and the aperture on the "anterior" end of the shell because when the snail moves it drags the shell behind it, apex rearward. Note that the "spire" is that part of the shell above the last turn of the shell or "body whorl". The "suture" is the line of junction between one whorl and the next; a "shoulder" is a more-or-less small platform on the whorl just below the suture.
The pictures below show a couple of the consequences of coiling in the gastropods. On the left is a cutaway view of Busycon carica from the southeastern U.S., showing how the coiling of the aperture around a central axis leaves a solid axial structure on its inside edge. This is called the "columella". The bottom (anterior) view of an unidentified top shell from Nuweiba, Egypt on the right shows how the last whorl sometimes leaves a cavity, the "umbilicus", at the anterior end of the columella. In other cases this cavity is absent or is covered with extra shell material called an "umbilical plug" (see Polinices duplicata at the top of the page).
Shell form is highly variable in the Gastropoda but the simplicity of shell growth means that many taxa, even unrelated ones, have arrived at similar overall shapes. Thus shell shape has only limited utility as a classification and identification tool. However, at the specific level, descriptions of snails often mention the shape, so a few terms and examples are worth knowing.
The most primitive snails have comparatively low-spired shells like the tops (below left and above right) called "trochiform" after the genus Trochus. High-spired shells like Terebra above are called "turriform" after the genus Turritella. Polinices (above) is "ovoid". Busycon (above left) is "fusiform". Conus (below right) is "coniform". The limpets and false limpets (bottom 3 pictures) have low, cone-shaped shells called "patelliform".
