482 THE INVERTEBRATA 



the surface of the foot is clothed with ciUa, 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 

 contraction. 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 . 3 84 B ) at their tip . Both are hollow 

 and have attached to the inside of the tip a muscle whose contraction 

 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 round hole 

 on the right side which is the aperture of the mantle cavity or pneumo- 

 stome. In the marine gasteropods the mantle cavity has a wide open- 

 ing to the exterior, though a part of the mantle border {siphon) is 

 modified to form a special channel by which fresh water for breathing 

 may be drawn in by the action of the cilia clothing the gill. But in the 

 air-breathing pulmonates where the cavity is converted into a lung, the 

 injury of delicate respiratory tissues by evaporation must be avoided, 

 and a pumping mechanism for renewal of air established. The re- 

 striction of the respiratory aperture is one of the necessary modifica- 

 tions. If a section is drawn across the lung of a snail it will be seen 

 that the mantle forms the roof of the cavity and is covered with ridges 

 in which run pulmonary veins converging towards the auricle. The 

 floor of the cavity is arched and has a layer of muscles, which con- 

 tract rhythmically. When they contract, the arch flattens and air is 

 drawn in and at the limit of contraction a valve slides across the 

 pneumostome. When the muscles relax, the cavity decreases in size 

 and exchange of gases with the blood in the roof vessels is facilitated 

 by the increase of pressure of the contained air. Then the pneumostome 

 opens and air is expelled; the subsequent contraction of the floor 

 muscles brings in a fresh supply. This "breathing" is not so regular 

 or so frequent as in a vertebrate ; moreover, it may cease altogether in 

 the winter when the snail hibernates. 



In dissection, the collar is cut and the roof of the mantle cavity 

 turned back so as to show the pericardium enclosing the ventricle and 

 single auricle, and the kidney, which is a yellow organ consisting of a 



