MOLLUSKS 

 Chapter 6 



Shells of snails and of clams or mussels have apparently always interested 

 man. Before Columbus arrived, the American Indians used shells or objects 

 made from them as ornaments and as a medium of exchange. Clam valves 

 were in wide use as spoons and scrapers. At the present time the collecting 

 of shells is a wide-spread hobby, and almost all of us who visit the sea-shore 

 gather some of the more attractive specimens. 



The collector, fortunately, need not confine his efforts to marine shells, 

 but can find a multitude of varieties both on land and in fresh water. Indeed, 

 before sewage and factory wastes had destroyed the life in many of our rivers, 

 America could justly claim to have more forms of fresh-water moUusks than 

 any other area in the world. The Coosa River in Alabama became world 

 famous among scientists because of the number of species of clams and snails 

 peculiar to it. 



The ecological problems connected with the distribution of mollusks have 

 received much and deserve more study. The famous work of J. T. Gulick on 

 the land snails of Hawaii, where each valley was found to have a different 

 variety of tree snail of one genus, Achatinella, with a degree of difference pre 

 portional to the distance between valleys, has important bearing on our theories 

 of species forming. The distribution of fresh-water clams or mussels is an ex- 

 cellent index of the condition of the streams, since clams cannot travel far 

 from place to place as the circumstances of their environment change, as do 

 fishes and other aquatic animals. 



The taxonomist naturally considers the whole animal rather than the shell 

 alone, and the finer points of classification are based on anatomical studies of 

 the soft parts, such as the gills and reproductive organs of clams and snails 

 and the rasp-like tongue or radula of the snails. Although all this study is es- 

 sential to the establishment of classification, once the taxonomic position has 

 been determined an authority can usually identify the animal by the shell 

 alone. The present keys are an attempt to make it possible for the beginner 

 to identify the specimens he may find from an examination of the more evident 

 structures. 



Most snails carry a one-piece shell, spirally coiled around a central axis 

 called the columella. A few small fresh-water snails, like some of their marine 

 relatives, have lost the spiral coiling and developed a pyramid-like or cone- 

 like shell. Several land genera have almost or quite dispensed with their shells 

 and are commonly called slugs. Most of these slugs have marks on their backs 



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