THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 29 



(Fig. 16) are composed of several thousand cells, arranged 

 in a single peripheral layer about the hollow center of 

 the ball. The cells are ovoid, and each is provided with 

 two long flagella which pro- 

 ject out into the water. The 

 lashing of the thousands of 

 the flagella give the ball- 

 like colony a rotary motion. 

 The cells are held together 

 by a jelly-like intercellular 

 substance and are connect- 

 ed with each other by fine 

 protoplasmic threads which 

 extend from the body pro- 

 toplasm of one cell to the 

 cells surrounding it. When 

 the colony is full grown and 

 ready to reproduce itself 

 certain cells of the colony 

 undergo great changes. 

 Some of them increase in 

 size enormously, having re- 

 serve food material stored 

 in them, and they may be 

 called the egg cells of the 

 colony. Reproduction may 

 now occur by simple divi- 

 sion of one of these great 

 egg cells into many small 

 cells, all held together in a 

 common envelope. These 

 form a daughter colony 

 which escapes from the 

 mother colony and by growth and further division comes to 

 be a new full-sized colony. Or reproduction may occur in 

 another, more complex, way. Certain cells of the colony 



B 



FIG. 16. A, Volvox minor, entire colony 

 (from Nature). B, C, and D, reproductive 

 cells of Volvox globator. 



