THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 41 
23. Differentiation of the body cells—In /Tydra we have 
the beginnings of complexity of structure carried a step 
further than in the sponges. The division of labor among 
the cells composing the body is more pronounced, and the 
structural modification of the different cells to enable them 
better to perform their special duties is obvious. Some of 
the cells of the body specially devote themselves to food- 
taking; some specially to the digestion of the food; some 
are specially contractile, and on them the movements of 
the body depend, while others are specially irritable or 
sensitive, and on them the body depends for knowledge of 
the contact of prey or enemies. In the lasso cells—those 
with the stinging threads—there is a very wide departure 
from the simple primitive type of cells. There is in Hydra 
a manifest differentiation of the cells into various kinds of 
cells. The beginnings of distinct tissues and organs are 
foreshadowed. 
The individuals of Hydra live, usually, distinct from 
each other. There is no tree-like colony, as with the sponges. 
But most of the other polyps do live in this colonial manner. 
The new polyps which develop as buds from the body of 
the parent do not separate from the parent, but remain 
attached by their bases. They, in turn, produce new 
polyps which remain attached, so that in time a branching, 
tree-like colony is formed. 
24. Meduse or jelly-fishes—Most of the other polyps 
differ from Hydra also in producing, in addition to ordi- 
nary polyp buds, buds which develop into bell-shaped struc- 
tures called medus@ (Fig. 20). These medusz consist of a 
soft gelatinous bell- or umbrella-shaped body, with a short 
clapper or stem which has an opening at its free end. 
From the edge of the bell or umbrella four pairs of tenta- 
cles project. The meduse usually separate from the parent 
polyp and live an independent, free-swimming life. These 
are the beautiful animals commonly known as jelly-fishes. 
The meduse or jelly-fishes produce special reproductive 
