GASTEROPODA 555 



the order Pulmonata to which Helix belongs is the most speciaHzed 

 and probably the latest developed division. Helix is a terrestrial 

 animal breathing by a kind of lung, while the majority of gasteropods 

 are marine animals breathing by gills, and besides the complications 

 which this involves, the reproductive system is hermaphrodite with 

 the most elaborate provision of glands and ducts which serve to 

 produce eggs well stored with nourishment and are arranged so as 

 to assure cross-fertilization. In the account of Helix which follows 

 an attempt is made to distinguish clearly between the purely gastero- 

 pod features and the adaptive features which belong to the Pulmonata. 



The body of a snail is composed of three regions, the head, foot and 

 visceral hump. The visceral hump is all that part which is covered by 

 the shell when the animal is expanded, while the head and the foot 

 make up the remainder outside the shell. There is no boundary be- 

 tween the latter two regions. The German zoologists refer to the 

 whole as the " Kopffuss " (the " head foot "), and this can be retracted 

 as a whole within the shell by the action of the columella muscle (Fig. 

 381). The foot is particularly characteristic of the Gasteropoda. It 

 possesses a flat ventral surface underlain by longitudinal muscle fibres. 

 If a snail is observed crawling up a pane of glass, a series of rippling 

 waves of contraction of very small amplitude are seen to pass regularly 

 over the surface of the foot. They are co-ordinated by the action of a 

 nervous network, such as occurs in the lower invertebrates (Fig. 1 10). 

 The gliding movement of a snail indeed resembles that of a turbel- 

 larian, and we actually find that in some marine gasteropods the 

 surface of the foot is clothed with cilia, which beat in unison, though 

 they are perhaps capable of inhibition by the central nervous system. 

 In most water snails, however, the foot moves by muscular contrac- 

 tion. To fit this kind of movement for passing over a hard dry surface, 

 there is in the snail a copious secretion of slime from a pedal (mucous) 

 gland which runs dorsal to the foot and opens just ventral to the 

 mouth. As soon as the slime emerges it is spread out as a smooth bed 

 of lubricating fluid along which the snail moves. 



There are two pairs of tentacles on the head of the snail. The first 

 are shorter and are supposed to be the seat of the sense of smell ; the 

 second bear a pair of simple eyes (Fig. 409 B) at their tip. Both are 

 hollow and have attached to the inside of the tip a muscle whose con- 

 traction turns them outside in. The mouth is a transverse slit just 

 ventral to the first pair of tentacles. On the right side of the body not 

 far below and behind the second pair of tentacles is the reproductive 

 aperture. On removing the shell, the junction of the visceral hump 

 with the rest of the body is seen anteriorly as a thickened collar which 

 is the edge of the mantle and the seat of secretion of the principal 

 layers of the shell. It is fused to the head of the snail except for a 



