202 PRACTICAL ZOOLOGY 



by a poison called hypnotoxin which is injected into them by 

 the tube. 



Two kinds of smaller nematocysts are also found in Hydra. 

 One of these is cylindrical and contains a thread without barbs 

 at its base; the other is spherical and contains a barbless thread 

 which, when discharged, aids in the capture of prey by coiling 

 around the spines or other structures that may be present 

 (Fig. 120, E). 



Division of Labor among Cells. — From a zoological 

 standpoint Hydra is of special importance because it is one of 

 the simplest of all the Metazoa or many-celled animals and gives 

 us an excellent opportunity to study the division of labor 

 among the different parts of the body. 



The bodies of all complex animals including man are not con- 

 tinuous masses of the fundamental living substance, the pro- 

 toplasm, but are broken up into millions of very small parts 

 called cells. Each cell is separated from every other cell by 

 means of partitions called cell walls. The term cell was first ap- 

 plied to these units of structure because when they were first 

 noticed in cork they reminded their discoverers of the cells of 

 monks in a monastery. These cells of which the bodies of ani- 

 mals are built up differ in size and shape, but as a rule the size 

 of the animal depends on the number of cells rather than upon 

 their size. The body of Hydra contains thousands of these cells 

 arranged in two layers as indicated in Figure 119. 



The Ectoderm of Hydra. — The outer layer, the ectoderm, 

 is primarily protective and sensory, and is made up of two prin- 

 cipal kinds of cells: some are shaped like inverted cones, and 

 possess long contractile fibrils at their inner ends (Fig. 120, A); 

 these enable the animal to expand and contract. The others 

 lie among the bases of these muscular cells; they give rise to 

 the three kinds of nematocysts or stinging cells and to nerve 

 cells and germ cells. 



The Entoderm of Hydra. — The entoderm, the inner layer 

 of cells, is primarily digestive, absorptive, and secretory. The 



