320 CHARACTERS OF INVERTEBRATE ANIMALS 



seen, lying, however, well over on the right side, while on the 

 left there is a single gill with the comb-like shape characteristic 

 of the order, and running alongside it a projecting ridge, the 

 water-testing organ (osphradium). The heart is placed imme- 

 diately behind the gill, and has but one auricle, placed in front 

 of the ventricle, which is not folded round the intestine as in 

 the Ormer. It is indeed exceptional for the intestine of a snail 

 to run through the heart, though it is characteristic of bivalve 

 molluscs. There is but one kidney in the Periwinkle, opening 

 into the back of the mantle-cavity, on the left-hand side. The 

 suppression of one gill, auricle, and kichiey is believed to be one 

 result of the twisting of the body, though exactly why is uncertain. 

 They have perhaps been subjected to pressure, and so to speak 

 squeezed out of existence. The twisting of the visceral loop in 

 the nervous system is another result of the coiling of the body, and 

 this is easily understood. 



Two species closely related to the Periwinkle are common 

 on British coasts. In one (Littorina rudis] the coiled apex or 

 spire of the shell is very short. The other (L. obtusata) is a 

 small form, varying in colour from greenish-brown to orange- 

 yellow, and entirely devoid of a projecting spire, the apex of the 

 shell being rounded off so as to make the general outline of the 

 shell spheroidal. It is common on the brown sea- weed (Fucus) 

 with which 'tween-tide rocks are often thickly covered. 



The River-Snail (Paludina), common in the streams of this 

 country, is something like the Periwinkle in general shape, but 

 it is a good deal larger and the shell is much thinner. 



Living side by side with the Periwinkle on our rocks will 

 be found the Purple Snail (Purpura lapillus), with a dense angular 

 white shell extremely unlike the rounded covering of the former 

 species. It belongs to a different family, and is a good example 

 of a carnivorous sea-snail. The mouth of the shell is notched 

 at its front end, i.e. the end away from the spire, for the trans- 

 mission of the siphon, a spout-like prolongation of the mantle 

 by means of which water enters the mantle-cavity. The Purple 

 is one of the forms in which the pharynx with its rasping organ 

 is situated in the end of a long proboscis, that is retracted when 

 not in use. The typical genus of this particular family is Murex, 

 many of the tropical species of which possess extremely beautiful 

 shells, covered with long spines and having the front angle of 



