REGULATION OF SPIRAL GROWTH IN PLANORBID GASTROPODS

Extensive experimentation has been performed on the planorbid Planorba rius metidjensis in order to determine which mechanism allows the snai l to coil its shell regularly. Individuals of this species, like all P lanorbidae, are permanently active and secrete their shells while craw ling on the substrate. Experiments consisted of attaching weights to e ither side of the shell (which is carried almost vertically) in an umb ilical position; these weights cause the shell to fall towards the sub strate on the loaded side. It can be demonstrated, qualitatively and q uantitatively, that during further growth the shell tube deviated init ially (i.e. within the first half whorl after loading) towards the loa ded side. In a later stage, when the animal is able to re-balance the shell-load complex by muscular activity, the shell tube gradually devi ates away from the loaded side. This behaviour is to be expected if, a fter loading, secretion of the shell continued with the aperture paral lel to the substrate and forming a constant angle with the direction o f growth. The main implication is that in normal conditions the living posture largely controls the correct coiling of the shell. Minor expe riments made with another planorbid species, Gyraulus laevis, confirm these conclusions. The growth pattern of planorbids requires that the snail has constant information on the orientation of the shell with re spect to the substrate. This is permitted by the particular physiologi cal ecology of this group, members of which, unlike terrestrial gastro pods, are permanently active.