review of the biochemistry of shell pigments has been made by Alex Comfort (1951).
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 specimens of the Horse Conch, the Queen Conch (Strombus 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 separate 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 Mya 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 manner 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 developed 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
