CHAPTER LXXVIII 



SOCIAL LIFE OF ORGANISMS 



457. Self-sufficient individuals. Among the lowest forms of 

 plants and animals each individual is quite independent of its 

 neighbors, as we may see in the case of the ameba, the Para- 

 mecium, the green slime, the various bacteria, and so on. 



Fig. 227. Diagram of sponge structure 



A sponge is a colony of cells arranged about hollow spaces, a, which are connected with 

 the surrounding water by means of hollow channels, l>, carrying currents inward, and by 

 means of other channels, c, carrying currents outward through larger tubes, or " sewers," d. 

 The currents are produced by the constant vibration of cilia projecting into the spaces, 

 and they bring to the cells fresh supplies of food and oxygen, and carry away waste 



Among many of the algae the cells generally remain attached 

 to form long filaments, but there is apparently no physiological 

 connection, and a break in the filament does not affect the 

 activities of the severed portions. 



Among the more complex algae, such as the bladder wracks, 

 detached portions may continue to grow, since each portion de- 

 pends upon materials absorbed from its immediate surroundings 



430 



