32 THE CHICAGO ACADEMY OF SCIENCES. 



deep or shallow. Specimens of Galba emarginata mighelsi from Aroos- 

 took County, Maine, exhibit similar fluctuations, as do also examples 

 from some of the lakes of Michigan and Wisconsin. Cooke 4 cites 

 examples of the European Lymncca pcregra and Lymnaa stagnalis 

 from the salt marshes near the sea of Aral which show the effect of 

 a changing environment. These lakes are salt for several months 

 of the year and comparatively fresh during the remainder of the 

 year. The effect is to dwarf and otherwise modify the form of the 

 shell. 



In many localities in which the habitat consists of a rapidly flow- 

 ing river or a wave beaten, rocky shore, the spire is shortened up and 

 the aperture is greatly increased in size. This increase in the size of 

 the aperture is in direct response to the character of the environment, 

 the roughness of the water causing the animal to develop a larger 

 foot, and hence, a larger aperture, in order to overcome the pushing 

 and pulling power of the water; in other words, a larger foot surface 

 is essential in this kind of a habitat, for the preservation of the species. 



A change of habitat is thought by some conchologists to produce 

 some species. Hazay, for example, records Lymncea percgra from 

 the ova of ovata, and ovata from the ova of pcregra, by placing one 

 species in running water and the other in still water. 1 



Dr. Lewis many years ago 2 asserted that Galba palustris in the 

 Erie Canal became Galba catascopium when transferred to the Gen- 

 esee River. 



Variation in Lymnaea may be summed up as due to one or more 

 of the following external causes : 



Quality and quantity of food. 



Station in stagnant or quiet water. 



Station in rapid current or on wave beaten shore. 



Temperature. 



Chemical nature of fluid medium. 



The foregoing remarks clearly indicate that the environment plays 

 a notable part in the variation of the Lymnaeas and hence in the for- 

 mation of species, and it seems almost needless to state that when 

 collections are made great care should be exercised in carefully noting 

 the ecological data. 3 



C. LOCOMOTION. 



Locomotion in Lymnaea is accomplished by three principal meth- 

 ods. These may be called gliding, hunching and thread spinning. 



K^ooke, op. cit., p. 93. 



2 Amer. Journ. Conch., IV, pp. 2-4, 186S. 



3 See Semper's Animal Life, p. 439, for a discussion of variation under 

 various conditions. 



4 Mollusca, p. 85. 



