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. 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 Fl - 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 



ST or "%L 



