Formation of Siphons in Lamellibranchia

Abstract

THE mantle edge in the Lamellibranchia consists, as is well known, of three lobes. Of these the one adjacent to the shell (Fig. I, I) is concerned with the secretion of this, while the bounding periostracum is secreted as a thread from the base of the groove (PG) between this and the next lobe. This middle lobe (2) has sensory functions, bearing tentacles and in certain cases, for example, Pecten and Spondylus, eyes. The innermost lobe (3) is characteristically muscular. It contains radial muscles attached to the shell along the pallial line and, between upper and lower strands of these, other muscle running longitudinally. Action of these muscles, in conjunction with distension caused by pressure of blood, is responsible for the movements of this lobe, usually known as the velum or, better, the pallial curtain. By this means the entrance of water into the mantle cavity is controlled, for example, in Ostrea1. In extreme cases, such as Pecten and Lima, it also controls the direction in which water is expelled during the swimming movements2.