274 ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



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

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



r into a permanent palisade or lattice-work. 



Tlu- larm- mantle-flaps which hang down on each side of the Pelecypod 

 nurt along thi-ir ventral edges when the shell is closed. Commonly 

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

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

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

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

 gills and tor its exit, and are hence known as the inhalent and exhalent 

 openings (Fig. in, C i and <?). 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 inhalent and 

 exhalent vary greatly within the group, in their length which may be 

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

 'i 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 



ropoda (Kig. 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 



yp)d type 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 



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). 



.vide inner end of the funnel lies within the deep part of the mantle- 



y, 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 



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

 back through the water. 



tin- alimentary canal the most characteristic and peculiar 

 H - minus tongue-like organ present in the pharynx of Gastero- 



