HYDRA 



135 



they are spermaries, or testes; if they are more knoblike and are situated 

 nearer the base they are ovaries (Fig. 55). 



161. Internal Structure. — When studied by means of sections 

 (Fig. 55) in which the structure is brought out by appropriate staining, the 

 hydra is seen to bo made up of a body wall surrounding a large central 



Cuficl& 



Fig. 55. — Somewhat diagrammatic longitudinal section of a hydra, showing two buds 

 differing in age on the left, and a spermary and ovary on the right. Batteries of nemato- 

 cysts are to be observed on the tentacles. In the gastrovascular cavity the entoderm is 

 seen to be made up of flagellated cells, cells bearing pseudopodia, and gland cells. A deli- 

 cate cuticle covers the ectodermal layer. 



cavity known as the gasirovascular cavity, or enteron, which opens to 

 the outside through the mouth. The gastrovascular cavity also extends 

 outward into each tentacle, reaching nearly to the tip, though the canal 

 so formed is very narrow. 



The wall of the body and that of the tentacles is composed of two 

 layers of cells separated by an extremely thin sheet of noncellular material 



