Original paper
Gastropod torsion: predation and the opercular imperative
Stanley, S.M.
Neues Jahrbuch für Geologie und Paläontologie - Abhandlungen Band 164 Heft 1-2 (1982), p. 95 - 107
45 references
published: Dec 22, 1982
DOI: 10.1127/njgpa/164/1982/95
ArtNo. ESP155016401026, Price: 29.00 €
Abstract
Torsion in gastropods (snails) is the early developmental process in which the visceral hump and the shell that houses it are twisted counterclockwise through approximately 1800 with respect to the underlying head-foot, so that the anus is brought forward above the head. Torsion is not structurally linked to coiling of the shell, as evidenced by the presence of uncoiled conical shells in some torted limpets (cap-shaped snails) and of coiled shells in some untorted monoplacophorans ("presnails"). Torsion in primitive living gastropods is accomplished by the pattern of development of the larval retractor muscles (CROFTS, 1937, 1955). The righthand muscle, which appears first, passes asymmetrically from the shell to the head; by contracting after full development, it accomplishes torsion by twisting the visceral hump with respect to the head. The left-hand retractor, which develops secondarily, takes over the function of shell attachment in the adult animal. As a consequence of torsion, the orientation of the shell changes from the orientation known as exogastric to that known as endogastric.
Keywords
snails • visceral hump • shell • head-foot • anus • cap-shaped snails • conical shell • exogastric • endogastric