THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 99 
(Fig. 16) are composed of several thousand cells, arranged 
in a single peripheral layer about the hollow center of 
the ball. 
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 
The cells are ovoid, and each is provided with 
Fig. 16.—A, Volvox minor, entire colony 
(from Nature). B, C, and D, reproductive 
cells of Volvox globator. 
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 
