274 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1952 



servation, and permanent agriculture in the humid regions of this 

 country." 



IMPORTANCE OF LIVING SNAILS 



Biologically, snails have many important relationships in addition 

 to their effect upon their physical environment. They have long been 

 recognized as important elements in food chains. Living chiefly on 

 vegetation, they in turn become food for fishes and a great variety 

 of other animals and thus in an endless cycle they keep much of the 

 organic material of which their bodies are composed passing from one 

 incarnation to another and at the same time much of the material 

 from their dead bodies and their wastes are continuously bridging 

 the gap, back and forth, within the bodies of plants and animals. 



In their capacity as necessary hosts for the digenetic trematodes, 

 which live as parasites in all the classes of vertebrates, snails have 

 peculiar biological significance. 



It is an indisputable fact that food wastes passing through their 

 digestive tracts have significant effect upon the organic content of 

 the soil and of the water in which snails live, but there have been no 

 quantitative studies paralleling the classical work of Charles Darwin 

 on the effects of earthworms in the production of vegetable mold. 

 Interesting as these aspects of relations between snails and their en- 

 vironment may be, they will be omitted from the present discussion and 

 attention will be restricted to the role played by snails through the 

 addition of lime to the soil. 



RELATIONS TO SURROUNDINGS 



While some species of snails have very broad tolerance of conditions 

 under which they will live, there are many species which are sharply 

 limited by their physical surroundings or by their necessity for asso- 

 ciation with particular types of living things. Thus, among the water- 

 dwelling snails, some of the forms of the old genus Physa which 

 breathe through a lung sac can prosper in water that is heavily poluted 

 by sewage {Physa anatina) and at times become abundant even in 

 sewage-treatment plants (Brown, 1937). Other forms of the same 

 genus (e. g., Physa gyrina hildrethina) occur only in pools, while 

 still other closely related forms {Physa gyrina gyrina) are character- 

 istic inhabitants of more swiftly moving streams. Among the snails 

 that dwell on the land, there is even more pronounced limitation to the 

 conditions under which many species will live (Baker, 1939). One 

 species {Discus patulus) is invariably associated with woodlands in 

 which beech and maple trees are growing. This species is considered 

 as an invariable associate of what ecologists term the beech-maple 

 forest. It is not that Discus feeds upon either beech or maple, but the 



