CCELENTERATA. 



On agitation of the water, Hydra contracts its body and 

 tentacles till it becomes a round knob, but if left to itself it 

 will soon expand again to its normal condition. 

 Very often the body appears to fork into two 

 parts each of which has a ring of tentacles. 

 One of these is a bud which is destined later to drop off 

 the parent. 



Internal 

 Structures. 



Fig. 46. — Transverse Section of Hydra (Magnified). {Ad nat.) 



Endoderm 



Coelenteron 



If the animal be killed and preserved and cut into trans- 

 verse sections, a low-power examination of such sections 

 reveals the fact that the whole body is a hollow sac, the 

 internal cavity being known as the coelenteron. The wall of 

 the body is of two layers, the outer layer or ectoderfn and the 

 inner or endoderm^ between which is a thin supporting 

 lamella, the mesogloea. 



The coelenteron may occasionally contain the bodies of 

 small animals which constitute the food of Hydra. 



On examination with a higher power of the microscope 

 the endoderm cells prove to be arranged in 



s ogy. ^ single layer and the cells themselves are 

 considerably larger than those of the ectoderm. Each cell 

 contains a nucleus and a number of small bodies scattered 

 through its protoplasm. 



