72 COMPARATIVE ANATOMY 



the apex upward and the aperture towards the observer, the 

 aperture is on the right, and if one follows the whorls from 

 above downwards they are coiled round- to the right. Such 

 shells are called dextral. Most Gastropod shells are dextral, 

 but in some cases the spiral is reversed, and such shells 

 are called sinistral. 



A snail drowned in warm water will generally die expanded, and 

 it may then be twisted out of its shell, care being taken to detach 

 a stout band of muscle on the right side from the columella. 



The external features may now be examined. The mouth 

 opens in the middle line anteriorly on the ventral surface of 

 the head, and is overhung by an upper hp. There are two 

 pairs of cephalic tentacles. The anterior pair is placed low 

 down on the sides of the head above the corners of the mouth. 

 The posterior pair is longer, and is placed on the sides of the 

 head above and behind the anterior pair ; each bears an eye 

 at its extremity. Both pairs of tentacles can be invaginated 

 and withdrawn into the body by means of special muscles. 

 A round opening on the side of the head below the right 

 posterior tentacle is the generative opening ; a shallow groove 

 leads back from it along the dorsal surface. The collar is the 

 thickened edge of the mantle, which, in Helix, is not reflected, 

 so as to cover any part of the external surface of the shell. 

 The thickened mantle edge is firmly fused to the body-wall all 

 round the base of the visceral hump, except in one place on 

 the right side, where there is a large round aperture leading 

 into a spacious chamber, the so-called pulmonary cavity. In 

 Paludina the mantle edge is not fused with the body-wall 

 along the front and sides of the visceral hump, and conse- 

 quently there is a wide passage into a cavity lying between the 

 visceral hump and the mantle, which is evidently the mantle 

 cavity. Paludina is aquatic, and the mantle cavity contains a 

 gill or ctenidium, but Helix is terrestrial, has lost its ctenidium, 

 and the mantle cavity is modified to form a pulmonary chamber 

 suitable to aerial respiration. 



The anus is a slit-like opening lying below and to the right 

 of the large respiratory aperture, and close above it is a still 

 smaller excretory opening. The position of the anus on the 

 right anterior side of the body indicates clearly the distortion 

 undergone by the upper part of the snail's body. In laraelli- 

 branchs the anus is median and posterior, but in gastropods 



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