MOLLUSCS. 



105 



Fig. 98. —Peri- 

 winkle {Litto- 

 rina liton a). 



The periwinkles, with their solid globular shells and their strictly vege- 

 tarian habits, contrast strongly with the forms just described. We have 

 several small species natives of our coast, but within the last few years 

 the genuine periwinkle of Europe has become acclimatized on our shores 

 How it first gained a foothold is uncertain, but once here it has rapidly 

 increased in numbers, so that now from Nova Scotia to New York it is the 

 most abundant mollusc. In 1857 it was first noticed by Mr. Willis, at 

 Halifax ; in 1870 it appeared on the shores of Maine; in 1872 

 it had reached Massachusetts ; and in 1874 it had passed Cape 

 Cod. It now occurs as far south as the New Jersey coast, but 

 it is probable that temperature and other causes will soon put 

 a limit to its further southward spread. 



The generic name of the periwinkle is Littorina, a deriva- 

 tive from the Latin word littus (the shore) ; and most appro- 

 priate it is, for these forms are pre-eminently littoral in their 

 habits. When the tide goes out, rocks, mud-flats, sea-weecl, piles, etc., are 

 covered with their slowly moving bodies. In Europe, the periwinkles are 

 extensively used as food, but here they have not yet come into favor, a 

 neglect doubtless clue to an ignorance of their qualities. The orthodox 

 way of eating them is to boil them and then extract the meat from the 

 shell by means of a bent pin. They taste much like clams, but are much 

 more delicate and tender. Their only fault is the small amount of meat 

 in proportion to the shell. 



In fresh water we find a group of pond-snails much like the peri- 

 winkles in structure, but, unlike them, of no value as food. They 

 are almost solely confined to the temperate regions, the United States 

 having many species. In all the shell is a dirty olive- 

 green, and all bring forth their young alive. The 

 generic name, Paludina, from the Latin palus (a marsh), 

 is suggested by their habits. The somewhat closely allied 

 Melanians, which are also inhabitants of fresh water, 

 derive their name from the dark brown or even black 

 shells, which are of a long conical shape. They are 

 especially abundant in our Western rivers, where the 

 variety of forms almost equals that of the fresh-water 

 mussels, and, as in the case of that group, has given rise 

 to an almost endless series of so-called species. 



Of a somewhat similar shape are a group of marine 

 shells which are parasitic or semi-parasitic in their habits, all occurring 

 on echinoderms and molluscs. Some merely live on the surface of the 

 host, but Professor Semper, who spent years in the Pelew Islands, 



Fig. 99. — Pond - snail 



{Palud I, with its 



body extended, as 

 when creeping. 



