Expansion of the Foot in Bullia (Gastropoda)

Abstract

SANDY-BEACH gastropods of the family Naticidae and of the genus Bullia (family Nassariidae) are capable of a remarkable expansion of the foot. So great is this expansion that it has often been suggested that the turgor of the foot is not due to hæmolymph alone but also to water from the surrounding medium which is supposedly introduced into the foot during expansion and expelled during contraction. Schiemenz^1,2 attempted to show such a mechanism in Natica and claimed to have demonstrated the existence of pores opening into an “aquiferous system” which was completely separate from the blood sinuses. Though these findings were accepted by a number of workers, including Gilchrist³, they have never been corroborated, and most subsequent authors have assumed, as did Pelseneer⁴, that the volume of hæmolymph is quite adequate for expansion of the foot in all gastropods. Nevertheless, a small body of opinion has tended to the view that in certain marine gastropods expansion of the foot may be brought about partly by the introduction of sea-water. Morris⁵, working on Uber, has tried to resolve the controversy by measuring the volume of fluid given off when the animal retracts and comparing it with the volume of water contained in the “free space” and elsewhere when the animal is expanded. Her results are unfortunately inconclusive and her observation that the expelled fluid often contains “blood-corpuscles” adds a complication to the problem.