SNAILS AND THE SOIL — VAN CLEAVE 275 



conditions of soil and of climate favorable to these trees are at the 

 same time the set of conditions demanded by Discus. There are many 

 other snails which are usual associates of particular kinds of plants 

 and animals because the latter provide the natural food for the snails. 

 One of the commonest woodland snails of central Illinois is Mesodon 

 thyroidus which will feed upon a wide variety of vegetation but be- 

 comes particularly thrifty and abundant where woods nettles grow 

 in profusion (Foster, 1937). Some land snails have feeding habits 

 wholly different from that just mentioned. For example, Haplotrema 

 concavum is carnivorous and lives largely upon the flesh of other 

 snails. Physical surroundings are less important in the life of the 

 last-mentioned species, just so long as a supply of snails is available 

 to serve as food. 



Many land snails are indicators of the physical conditions under 

 which they live (Baker, 1921). Their soft bodies are best served 

 by relatively high atmospheric moisture so that moist ravines sheltered 

 from the direct rays of the sun are particularly favored habitats and 

 many species become especially active at night and during rains. 

 Some of the land snails (as, for example, some species of the genus 

 Succinea) are so exacting in their demands for moisture that they 

 live a truly amphibious existence, remaining only a portion of the 

 time out of water and always close enough to water to be able to 

 return to it if they become too dry. Even when snails live in exposed 

 situations, such as in open fields and along railroad embankments, 

 or under desert conditions, they tend to conserve their moisture by 

 crawling under objects lying on the soil, or even live in the roots of 

 the vegetation. Further, many land snails are able to cut down 

 evaporation from their bodies, when inactive, by secreting a tough 

 membrane, the epiphragm, effectually sealing over the aperture of the 

 shell. 



Whether in the water or on dry land, the shells of dead snails 

 often resist destruction for a long period of time and under proper 

 conditions they accumulate in extensive deposits (pi. 1). By the 

 study of these deposits, which often represent forms of bygone 

 geological age, the student familiar with the habits of living snails 

 is often able to analyze the conditions under which each species 

 probably existed. In fact, at times it has been possible for the special 

 student (Baker, 1937) to postulate the general temperature range and 

 something of the ecological conditions of the environment under 

 which the animals which produced the dead shells lived, even though 

 that might have been at some remote time, several thousand years 

 in the past. With a considerable degree of confidence, Frank C. Baker 

 (1937) postulated the climatic conditions and the ecological associa- 

 . tions for some of the molluscan faunas of the ice age. In his studies 



