48 ANIMAL LIFE 
are very sensitive, and the polyps bearing them have for 
special function that of feeling or being sensible of stimuli 
from without. They are the sense organs or sense indi- 
viduals of the colony. Finally, there are two other kinds 
of structures or individuals which produce the special 
reproductive cells for the perpetuation of the species. 
These are the modified medusa individuals, and one kind, 
larger than the other, produces the active sperm cells, 
while the other produces the inactive egg cells. 
2%. Increase in the degree of complexity.—In the corals, 
sea-anemones, and jelly-fishes there is plainly much more 
of a division of labor among the various parts of an indi- 
vidual and much more modification of these parts—that is, 
much more structural complexity than among the sponges 
and Hydra. And these, in their turn, are more complex than 
are the colonial Protozoa, the Volvocine. There is a great 
difference in degree of complexity among the slightly com- 
plex animals. But the various groups of these animals 
which we have studied can all be arranged roughly in a 
series beginning with the least complex among them and 
ascending to the most complex. And in this series, and 
in the always accompanying division of labor among the 
different parts, the gradual increase in complexity is beau- 
tifully shown. 
From an animal composed of many structurally simi- 
lar cells, each cell capable of performing all the life pro- 
cesses, we pass to an animal composed of cells of a few 
different kinds, of slight structural diversity. Each kind 
of cell devotes itself especially to a certain few life pro- 
cesses or functions. Next we find an animal in which the 
cells of one kind are specially aggregated to form a single 
part of the body which is specially devoted to the perform- 
ance of a single function. This diversity among the cells 
increases, this aggregation of similar cells to form special 
parts or organs increases, and the division of labor or 
assignment of special functions to special organs becomes 
