‘ THE SPONGES 21 
In such associations as Volvor an important step has 
been taken beyond that of Pandorina, for there is a division 
of the labors of the colony among its various members, 
some acting as locomotor cells while others are germ-cells. 
These are now so dependent one upon the other that they 
are unable to exist after separation from the main com- 
pany, just as a part of the squirrel is incapable of leading 
an independent existence. A higher type of organism has 
thus arisen intermediate between the simple one-celled 
animals and those of many cells, especially the sponges—a 
relation which is more readily recognized after.an examina- 
tion of the latter. 
23. Development of the sponge—As with all many- 
celled animals, the sponge begins its existence as an egg, 
in this case barely visible to the sharp unaided eye. Fer- 
tilized by its union with a sperm cell, development com- 
mences, and the first apparent indication of the process 
will be the division of the cell into two halves (Fig. 11, A, B). 
Each half redivides into four, these again into eight cells, 
and this process is repeated, giving the young sponge the 
general form of Pandorina. The divisions of the cells 
still continue and result in the formation of a hollow 
globe of cells (called the dlastula, Fig. 11, E, F) similar 
to Volvoz, and at this point the young larva leaves the 
parent. 
The next transformation consists in a pushing in of one 
side of the sphere, just as one might press in the side of a 
hollow rubber ball. The depression gradually deepens, and 
finally results in the formation of a two-layered sac known 
as the gastrula (Fig. 11, G). At this stage of its existence 
the sponge settles down for life in some suitable spot, by 
applying the opening of its sac-like body to some foreign 
object. In assuming the final form a new mouth breaks 
through what was once the bottom of the sac, canals per- 
forate the body wall, a skeleton is developed, and the char- 
acteristic features of the adult are thus attained. 
