3IO Aquatic Societies 



practically all entomostraca. Certain hardy species of 

 Cyclops and Chydorus appear to be least sensitive to 

 stagnation conditions. The insect Corethra, (fig. 183) 

 is remarkable for its ability to live in the depths, where 

 practically no free oxygen remains. 



Age appears to be another factor in vertical distribu- 

 tion. On the basis of his studies of the Entomostraca 

 of Lake Mendota. Birge ('96) has formulated for them 

 a general law of distribution, to the effect that (i) 

 broods of young appear first in the upper waters of the 

 lake (quite near the surface) ; (2) increase of population 

 results in extension downward, and the mass becomes 

 most uniformly distributed at its maximum develop- 

 ment; (3) with decline of production there is relative 

 increase of numbers in the lower waters. 



Perhaps this shifting downward merely corresponds 

 to the wane of vigor and progressive cessation of swim- 

 ming activities with advancing age. 



In the case of many plants spore development or 

 encystment may follow upon a seasonal wave of produc- 

 tion, with a resulting change in vertical distribution. 

 Filamentous blue-green algce develop spores. The 

 ordinary vegetative filaments are buoyed up in part by 

 vacuoles within the cells, that lessen their specific 

 gravity; but spores lack these. Hence the spore-bear- 

 ing filaments settle slowly to the bottom, and may be 

 found in numbers in the lower waters ere they laave 

 reached their winter resting place. Dinobryon main- 

 tains itself at the surface in part by means of the lash- 

 ings of its fiagella, but when its cells encyst, the flagella 

 stop, and the fragmenting colonies slowty settle. Thus, 

 both internal and external conditions have much to do 

 with vertical distribution. In general it may be said 

 that during their period of highest vegetative activity 

 all plants are necessarily confined to surface waters; 

 that most animals are closely associated with them. 



