l82 ELEMENTARY ZOOLOGY. 



this outer wall on the one hand or of this internal cavity 

 on the other. Digestion and circulation take place in the 

 cavity (gastro-vascular) , and its entodermal cells absorb 

 the food. Respiration, or the taking up of oxygen and 

 the giving up of carbon dioxide, take place by means of 

 the whole body surface. There are muscle fibers, both 

 circular and longitudinal in the body wall. These are 

 better developed in the higher forms. The nervous cells 

 are either scattered over the body or grouped at the free 

 end near the mouth and tentacles, or in the jelly-fishes 

 around the margin of the umbrella. If there are any 

 sense organs they are in the same regions. 



Several things show us how lowly is the organization of 

 this branch. We have seen how readily they bud and 

 form new individuals. This shows that all the tissues 

 are highly reproductive. This is not true of the most 

 differentiated animals. Furthermore, numerous experi- 

 ments on different members of the branch show us that 

 they have great power of regenerating lost parts when 

 these are cut away. The Hydra and other polyps may 

 be cut into two or more pieces, and under favorable 

 conditions each piece may become a perfect animal with 

 all its proper parts. This again shows low organization. 



200. The Place of the Ccelenterates in Nature. The food 

 of the coelenterates consists largely of. organic matter 

 broken up by the waves, and of small,- frequently micro- 

 scopic, organisms. Since they are attached, their food 

 must be brought to them either by currents and waves or 

 by the motions of their prey. They must live therefore 

 in regions where food is plentiful. There is no place in 

 the whole earth where food is more plentiful than at the 

 bottom of the ocean near the shore. All the passive 



