II. ECOLOGY OF THE LYMN^AS. 



a. GENERAL ENVIRONMENTAL CONDITIONS. 



The Lymnseas inhabit all varieties of bodies of water, from the 

 small transient pool to the great lakes. While the family as a whole 

 is thus widely distributed, many of the species are quite restricted in 

 their habitat. Some species (stagnalis appressa, for example) live 

 in marshy, reed-bordered lakes and ponds, preferring open patches 

 of water near the shore in early spring and venturing out into the 

 larger body of water in the fall. Another group of species represented 

 by palustris prefers stagnant ponds where there is considerable decay- 

 ing vegetation. They may be found crawling over the bottom of the 

 pond or pool, on the vegetation or on any other submerged object. 

 Some of the species, like caperata, live in small running streams, in 

 stagnant overflows of creeks, or in small pools which dry up in periods 

 of drought. The small species, like parva and humilis modicella, live 

 on the margins of streams, ponds and creeks, on the mud bordering 

 the shore or on sticks, stones and various kinds of debris. These 

 species are seldom found in the water, unless recently submerged by 

 a rise of the water, their natural .habitat seeming to be just above 

 high-water mark. When kept in captivity they invariably crawl out 

 of the aquarium and may be found on tables, chairs or even the floor. 

 A few Lymnseas, like apicina and emarginata, prefer a rocky habitat, 

 either in running water or where there is a decided wave action as 

 on the beach of Lake Superior in Ontonagon County, Michigan, and 

 on the sandy shores of the larger inland lakes, like Tomahawk Lake, 

 Wisconsin. So important has it become that exact data should be 

 secured concerning the environmental relations of animal groups that 

 it has been thought necessary to illustrate a few of the more typical 

 and noteworthy habitats of a few species of Lymngea. As the central 

 United States is the metropolis of this family in North America, the 

 illustrations and examples are mostly taken from this region. 1 



Galba caperata (Say). Plates L; LI, figure 1. 



This species, in company with Aplexa hypnorum and Sphcerium occidentale, 

 lives in small streams, overflowed portions of small rivers and creeks, and in 

 small transient pools in the woods. (Plate LI.) In the summer and fall these 



1 It is not to be supposed that the different species occupy only stations 

 like those pictured. They will be found in any locality furnishing a more or 

 less similar set of conditions. 



