Marsh Societies 341 



It is some of these animals of the pond bottom that 

 give to the littoral region its great extension out tinder 

 the open waters of the lakes. It is only a few members 

 of the population that are able to endure conditions in 

 the depths far out from shores. These are such as: 



Small mussels of genus Pisidium. 



Mayfly nymphs of the genus Hexagenia. 



IMidge larvas of the genus Chironomus. 



Caddis- worms in the cylindric cases of sand, not yet 

 certainly identified, etc. 



The larger animals of the pond that belong neither 

 to surface nor bottom and that correspond to neither 

 plancton nor necton of the open water may be grouped 

 as: 



(i). Climbing forms fmost of which can swim on 

 occasion) , such as the scuds (Amphipods) , the nymphs 

 of dragonflies such as Anax, of damselflies such as Lestes 

 and Ischnura, of mayflies such as Callibaetis, larvae of 

 caddisflies such as Phryganea and of moths such as 

 ParapomT^, mussels such as Calyculina, and many 

 leeches, entomostracans and rotifers. 



(2). Sessile forms such as hydras, sponges, bryzoans 

 and rotifers. 



II 



Marsh Societies. — We come now to consider the 

 associations of organisms in waters that are not too deep 

 for the growth of standing aquatics. Shoalness of 

 water and instability of temperature and other physical 

 conditions at once exclude from residence in the marsh 

 the plants and animals of more strictly limnetic habits ; 

 but it is doubtless the presence of dense emergent plant 

 gro^^i:h that most affects the entire population. This 

 gives shelter to a considerable number of the higher 

 vertebrates, and these rather than the fishes are the 

 large consumers of marsh products. The muskrat 



