HYDRA 29 



through the bodies of fresh water. When the colony has 

 reached its full size a difference among the cells becomes 

 apparent. A few increase in volume to a marked de- 

 gree, owing to food material that they store up, and 

 their future development occurs along one of two lines. 

 Some divide repeatedly, and sinking into the interior 

 form daughter colonies that escape with the rupture of 

 the parent wall. Under other conditions certain cells 

 divide repeatedly to form bundles of slender flagellate 

 cells called sperms, any one of which may unite with a 

 particular sort of large cell or egg which then enters upon 

 essentially the same course of development as the one 

 just described. 



It is evident that Volvox lies a step ahead of Gonium 

 in regard to complexity. In the former there are 

 the beginnings of a division of labor among the com- 

 ponent cells based upon structural differences. Certain 

 cells taking no part in locomotion serve as reproductive 

 elements, while the great majority function in locomo- 

 tion, sensation, and feeding, but are wholly unable to 

 reproduce. The importance of this fact will appear with 

 greater clearness after we have examined another type 

 of still greater complexity. 



Hydra. — The last example that we shall examine in 

 its entirety is Hydra (Fig. 7), a freshwater animal re- 

 lated to the sea anemones, corals, and jelly-fishes of the 

 ocean. The body, brown or green in color, presents the 

 form of a slender hollow cylinder about one-fourth of 

 an inch in length. One end of the body is attached by 

 a sticky substance to some submerged leaf or stone, while 

 the other bears the mouth surrounded by five or six 

 slender outgrowths termed tentacles. The body wall, 

 throughout its entire extent, is composed of two layers 

 of closely grouped cells in which various types can be 

 distinguished. In the outer layer there are cells that 

 serve for protection, sensation, and motion, while special 

 nettle or stinging cells paralyze the small organisms 



