274 ZOOLOGY FOR MEDICAL STUDENTS chab. 



but in the majority of Pelecypoda this is not so, for the cihary brushes 

 have become replaced by bridges of solid tissue which knit the filaments 

 together into a permanent palisade or lattice-work. 



The large mantle-flaps which hang down on each side of the Pelecypod 

 meet along their ventral edges when the shell is closed. Commonly 

 there is present at the posterior end of the creature a space between 

 the two mantle-flaps, divided more or less distinctly into two parts, a 

 ventral and a dorsal : these serve respectively for the indraught of 

 water into the mantle-cavity caused by the ciliary movement of the 

 gills and for its exit, and are hence known as the inha^nt and exhalfint 

 openings (Fig. in, C, i and e). When these openings are, as is frequently 

 the case, demarcated by complete fusions of the mantle-edges their lips 

 may be prolonged into tubular siphons. These siphons — inhalant and 

 exhal^nt — vary greatly within the group, in their length which may be 

 relatively enormous as in Teredo — the " Ship-worm " — whose burrows, 

 often seen in bits of timber washed up on the shore, are traversed 

 throughout their length by the siphons, and in the degree to which the 

 two siphons are independent or on the other hand fused together through- 

 out a less or greater extent of their length. 



The foot is seen in what is probably its primitive form in the 

 Gasteropoda (Fig. in, A, F) where it forms a flat creeping sole such as 

 may be seen in the case of an ordinary snail or limpet. In the Pelecypoda 

 the flat sole has been lost except in a few of the most archaic and 

 the lower boundary of the foot is now a blunt edge while the outline as 

 seen from the side is somewhat like that of a ploughshare (Fig. in, C, F). 

 This type of foot is in fact used for ploughing through sand or mud, the 

 Pelecypod tj'pe of mollusc being specialized for a burrowing existence. 

 In the most primitive Siphonopod — Nautilus — the foot has become 

 tongue-shaped and is rolled on itself, one edge overlapping the other so 

 as to form a funnel-shaped siphon. In the ordinary Siphonopods the 

 inrolled edges become in the course of development completely fused 

 together so as to make the funnel completely tubular (Fig. in, B, F). 

 The wide inner end of the funnel lies within the deep part of the mantle- 

 cavity, the mantle-flap fitting round it, and in the typical Siphonopods 

 being temporarily fixed to it by special fasteners. The mantle-flap is 

 muscular and the Cuttlefish is able to make it contract suddenly so as to 

 force the water contained in the mantle-cavity outwards through the 

 siphon in the form of a sudden jet, and cause the animal to shoot violently 

 back through the water. 



As regards the alimentary canal the most characteristic and peculiar 

 feature is a curious tongue-like organ present in the pharj'nx of Gastero- 



