Glass 2. Gastropoda. 295 



In the majority of the Gastropoda the shell is formed in the way 

 described above; there are, however, exceptions. In some forms {e.g., 

 Vermeius) it is coiled in quite an irregular spiral ; this is correlated 

 with its firm attachment to some foreign object ; the regular spiral 

 twisting of snail sheila is connected with the fact that a regular 

 spiral shell can be more easily carried than a long and straight or 

 an irregular one. In some forms, which are free-living whilst young, 

 but sessile later, the shell is, at first, regular, but then grows 

 straight or irregular. In others it is hardly possible to speak of a 

 tube, the whole shell is a simple basin, and the visceral hump, a soft 

 pad on the dorsal surface (Limpets, Patella). In many Gastropods 

 the shell and viscera are rudimentary, or altogether absent; the 

 viscera are then situated in the lower portion of the body. In some 

 cases, in which the shell is rudimentary, or feebly developed, it is 

 partially or completely enclosed in a fold of the skin. 



The visceral sac, for the most part, lies freely within the shell, but 

 it is firmly connected with it in several places, namely, where the 

 columellar muscle arises from the columella. This muscle lies 

 on the ventral side of the visceral sac, and thence runs into the lower 

 portion of the body, which it withdraws into the shell when the animal 

 is disturbed. 



In many of the Gastropods there is a plate of eonchiolin, or 

 oonchiolin and calcium carbonate, on the upper surface of the 

 hinder portion of the foot. When the whole animal is retracted 

 into the shell, the foot is drawn together in such a way that this 

 plate lies below, forming a lid or operculum. The operculum 

 is firmly fused to the animal beneath, and grows by the secretion 

 of new material from the .skin ; sometimes growth occurs in such 

 a way that the operculum displays a spiral line on its upper surface; 

 this, however, is only rarely the case. 



The tme operculum which is firaily imited to the animal, and fp-adually 

 increases in size, must not be confounded with the winter operculum 

 (epiphragma). The latter is usually as thin as paper, occasionally somewhat 

 thicker; and occurs, e.g., in species of Helix, being veiy thick and fii-m in 

 H. pomatia. When the animal is about to hibernate it withdraws into its shell 

 and the epiphragm is formed at the mouth. It consists of a hardened calcified 

 mucus, and is not attached to the animal, but at the end of the winter is 

 thrown off, a new one being f oi-med each year. 



The skin is soft and slimy ; the mucus is secreted by unicellular 

 glands which open in great numbers on the surface of the body. 

 In many of the Pulmonata, larger skin glands, which also secrete 

 mucus, are present. In the Prosobranchiata there is a peculiar 

 patch of glandular epithelium on the inner side of the mantle, the 

 so-called " m u c o u s -g 1 a n d ." In some Gastropoda, in addition 

 to mucus, these glands also secrete a fluid, "purple," which, under 

 the action of light becomes of a permanent violet colour. 



