3i8 



Aquatic Societies 



coid green algae. The herbivores are such short-Hved 

 crustaceans as Chirocephalus (see fig. 90 on p. 184) and 

 Apus, which have long-keeping, drouth-resisting eggs; 

 such rotifers as Philodina, remarkable for its capacity for 

 resumption of activity after dessication ; such insects as 

 mosquitoes. The carnivores are such adult water-bugs 

 and beetles as may chance to fly into them. 



Whether a population shall be able to maintain itself 

 depends on the continuance of favorable conditions, at 

 least through the period of activity of its members. 

 In these pages we shall give attention only to the life 

 of relatively permanent waters. 



Plants — The shoreward distribution of plants in 

 natural associations is determined mainly by two 

 hydrographic factors: (i) movement and (2) depth of 

 the water. It is directly related to exposure to waves 

 and to currents. Everyone knows the difference in 

 appearance between plants growing immersed in a quiet 



Fig. 188. The forefront of the Canoga marshes, where partly sheltered 

 from the waves of Cayuga Lake, clumps of the lake bulrush lead the 

 advance of the shore vegetation. 



