THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 41 



23. Differentiation of the body cells. In Hydra 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. Medusae 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 medusae (Fig. 20). These medusae 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 medusae usually separate from the parent 

 polyp and live an independent, free-swimming life. These 

 are the beautiful animals commonly known as jelly-fishes. 

 The medusae or jelly-fishes produce special reproductive 



