VEGETATION COMMUNITIES 



131 



all the species are relatively independent of the atmosphere and of the 

 bottom. 



c) Emerging vegetation association of bays. — Such situations as are 

 occupied by this association are found in bays and protected situations in 

 the larger lakes and represent a stage which is last in the history of a 

 lake. Water-lilies, water buttercups, and Myriophyllum are the prin- 

 cipal plants. Filamentous algae are usually very abundant. Logs, 

 sticks, and pieces of wood are not uncommon. 



On the under side of logs, we find such forms as the polyzoan 

 (Plumatella) and sponges (Spongilla sp.). On the under side of the water- 

 lily pads are usually numbers of Hydra together with great numbers of 



Fig. 82. — A muskrat's nest adjoining the lake border among the bulrushes on 

 sandy bottom. 



shelled protozoans and rotifers, especially sessile forms. Snails also are 

 common here (Segmentina armigera, Planorbis parvus, Physa gyrina and 

 integra, Planorbis campanulatus, and some species of Lymnaed). 



A large number of species of aquatic insects cling in the vegetation 

 with the abdomen near the surface of the water and secure air through 

 various anatomical arrangements which conduct it to the spiracles; the 

 most noteworthy of these are the water scorpion (Ranatra), the electric- 

 light bugs (Benacus and Belostoma), the predaceous diving beetles 

 (Dytiscidae) (99c), the water scavengers (Hydro philidae), and the water- 

 boatmen (Corixa). There are also a number of aquatic insects that are 

 not dependent upon the atmospheric air in their young stages. They 

 require, however, some object which reaches above the surface of the 



