Beiträge zur Torsion und Frühevolution der Gastropoden

Contributions to torsion and early evolution of the gastropods The phylogenetical model, which is presented here, is directed by trying to explain torsion of gastropods out of biomechanical conditions. It is based on morphological and functional reflexions upon the evolution of an ancestral mollusc out of an annelid-like segmented coelomate and further evolution of this to a monoplacophoran. Starting point of gastropod evolution is a high-arched monoplacophoran with serial pairs of dorsoventral muscles. There are always alternating one pair of vertically and one pair of obliquely running, crossed dorsoventral muscles. Evolution of higher arches causes a reduction of many dorsoventral muscles and a magnification of the pallial cavity. This gives place for a few pairs of massive gills instead of many serrated little monoplacophoran gills. A second consequence is a waist (“Taille”) between cephalopodium and visceral hump. It functions as an axis for at first little, with further muscle reductions greater torsional motions of the shell and the visceral hump. At last there exists only one pair of oblique and crossed dorsoventral muscles. This situation forces a fixation of the torted position, because it causes less energy consumption. There is also a lot of other advantages for the “torted gastropod”. The construction of a bilateral ancestor is the base for the evolution of recent asymmetric snails as a bundle of different phylogenetic lines.