Life of the Snails 21 



review of the biochemistry of shell pigments has been made by Alex Com- 

 fort (195 1 ). 



Rate of growth and span of life in gastropods vary according to the 

 species and ecologic conditions. The maximum age of marine species is very 

 imperfectly known. Undoubtedly many species live for only two, three or 

 four years. The common European periwinkle {Littorina littorea) found in 

 New England has been kept alive in captivity for twenty years. Large speci- 

 mens of the Horse Conch, the Queen Conch (Strojnbus gigas) and the 

 Cameo King Conch (Cassis) probably represent ten to twenty-five years of 

 growth. The nudibranch sea slugs are believed to be short-lived, and Aeolis 

 and Goniodoris have been shown to survive only into the second year. It is 

 quite likely, though, that the Aplysia seahares and the Bulla Bubble Shells 

 live for at least five years. 



The ultimate size of individuals in species in which the sexes are sepa- 

 rate may be influenced by the sex of the individual. In many groups, such 

 as the buccinid and Busycon Whelks, the Strombus conchs, periwinkles and 

 others, the shells of the females are always considerably larger. In the Pale 

 Lacuna Periwinkle (Lacuna pallidula), the females are from five to ten times 

 as large as the males. 



Considerable variation in size results from the diet of mollusks. It has 

 been experimentally shown that the Ovster Drill snails (Urosalpinx cinerea) 

 eating My a clam and oyster meat show the greatest increase in growth, while 

 those feeding on barnacles and Mytilus mussels show the least amount of 

 growth. It has also been found that snails of this species living in brackish 

 water grow to a larger size than those living in pure sea water. Colonies of 

 snails exhibiting these ecologic characters have been erroneously considered 

 new species by some workers. 



HOW THEY FEED 



The gastropods are much more imaginative in their selection and man- 

 ner of acquiring food than the bivalves and other mollusks. Unlike the 

 clams, most snails travel in search of their food. A great proportion of the 

 marine gastropods are carnivorous, but some are detritus feeders, others are 

 vegetarians, and a few, like their bivalve relatives, are suspension feeders. 



Among the flesh-eating snails, there have been many modifications in 

 the structure of the mouth parts, including the proboscis and the teeth. In 

 some the snout has remained very simple, and the snail merely pushes the end 

 of its mouth against its food and tears off bits with the tongue-like radula or 

 row of teeth. But in others a remarkably long, tube-like extension is devel- 

 oped which, when not in use, is retracted within the snout or head of the 

 snail. When a living Melongena Crown Conch is quickly picked up, one can 



