42 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 medusae bud off, 

 swim away, and finally produce new 



*-Z%2** P ol yp s - Th there is in the lile 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 

 medusae or jelly-fishes. Here the nerve cells are aggregated 

 in little groups arranged along the edge of the umbrella 



