49 ANIMAL LIFE 
cells, a single medusa producing only one kind of such cells 
—that is, producing either egg cells alone or sperm cells 
alone. The active sperm cells produced by one medusa 
find their way to an egg cell producing medusa, and fuse 
with or fertilize these egg cells. The 
fertilized egg develops into a small, 
oval, free-swimming embryo called a 
planula, which finally attaches itself 
to a stone or bit of wood or seaweed, 
and grows to be a simple cylindrical 
polyp attached at its base and with 
mouth and tentacles at its free end. 
This polyp gives rise by budding to 
new polyps, which remain attached 
to it, and gradually a new tree-like 
colony is formed. From this polyp 
or this colony new meduse bud off, 
swim away, and finally produce new 
Fra, 20. A medusa, Zucore. yolyps. Thus there is in the life of 
the polyps what is called an alterna- 
tion of generations. ‘There are two kinds of individuals 
which evidently belong to the same species of animal, or, 
put in another way, one kind of animal has two distinct 
forms. This appearance of one kind of animal in two 
forms is called dimorphism. We shall see later that one 
' kind of animal may appear in more than two forms; such 
a condition is called polymorphism. In alternation of gen- 
erations we have the polyp animal appearing in one genera- 
tion as a fixed cylindrical polyp, while in the next generation 
it is a free-swimming, umbrella-shaped medusa or jelly-fish. 
The polyps which are dimorphic—that is, have a polyp 
form of individual and a medusa form of individual—show 
more differentiation in structure than the simple Hydra. 
This further differentiation is especially apparent in the 
meduse or jelly-fishes. Here the nerve cells are aggregated 
in little groups arranged along the edge of the umbrella 
