22 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



thrown out through the mouth opening. The digestive cavity 

 extends out into the tentacles. 



In a prepared cross-section, the body of Hydra is seen to 

 be a hollow cylinder with walls made up of two well-defined 

 layers of cells with a thin non-cellular layer between them. 

 Some of the cells of the outer layer, or ectoderm, have contractile 

 basal processes running parallel to the long axis of the body. 



"W^ 33 ^ 



^ } 



/ ./ 



*' 

 ^"^v / ^" 



^$&~ 



FIG. 6. Hydra. Note two tentacles catching an insect larva; note the 

 budding young Hydra. (Natural size, one-sixth inch; from life). 



Like the muscle cells of higher animals these cells contract 

 under certain stimuli, and the whole body of the animal is 

 shortened, as we have seen. Among these cells, particularly 

 on the upper part of the body and on the tentacles, are small 

 stinging cells called nematocysts. The cells within which nema- 

 tocysts develop are called cnidoblasts, and each is provided at 



