THE LIFE OP 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 jj 

 certain cells of the colony ^&r /^/ (f 

 undergo great changes. C " /' *" 

 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. Eeproduction 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 FIG. 16. A, Volvox minor, entire colony 



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 



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

 cells of Volvox globator. 



