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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 haemolymph alone but also to water from the surrounding medium which is supposedly introduced into the foot during expansion and expelled during contraction. Schiemenz1,2 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 haemolymph 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.

THE study of quantitative collections of dead shells belonging to the lamellibranch Mactra stultorum Linne, for the year classes of 1952–1957 on the Dogger Bank, is yielding valuable information on the mortalities exerted by several classes of predators. The chief ones which are recognizable from the dead shells are the drilling prosobranch Natica poliana Forbes and the asteroids Asterias rubens Linne and Astropecten irregularis (Pennant). The prosobranch drills a hole through the shell and presumably feeds through this hole; the actual method of its feeding has not been described although its method of drilling has received considerable attention. Turner1 lists a full bibliography on the drilling mechanism. Very occasionally, live Mactra are caught, and more rarely, their dead shells, bearing an incompletely pierced bore-hole. It is presumed that the Natica was interrupted in its meal through itself being attacked by a larger predator. Even more interesting are some very rare instances of Mactra having sealed over the incomplete bore-holes on the inside, with an outgrowth of the nacreous layer. We have noticed such seals in both living and dead (at the time of collection) bivalves, in individuals ranging from 4 to 31 mm. Fig. 1 shows a typical sealed bore-hole. The area covered by the seal is very much larger than the bore-hole itself; this may be explained by the fact that, as has been observed, sand enters through the bore-hole and irritates a large area of the mantle membrane. Wasteful killing of very young Mactra by very young Natica seems to be indicated by shells which bear up to five bore-holes and still contain flesh.

The ability of marine prosobranchs (Nucella, Ocinebra, Natica) to drill holes in the shells of other mollusca, afterwards introducing their proboscis to suck out the soft parts of their victims, has been known by man during the past two thousand or so years1. Nevertheless, it has still not been decided whether this drilling takes place by mechanical means only or whether a secretion of acid from the pharynx, the salivary glands or (in the Naticidae) from the so-called boring gland takes part in the process. Schiemenz2, Hirsch3, Boettger4, and Ankel5,6 are all of opinion that secretion is active during drilling, whereas Fischer7 and Pelseneer8 feel convinced that drilling is carried out with the radula, that is, by mechanical means only.