Descriptive Zoology. 



The Ectoderm. The cells of the ectoderm are clear, and 

 more uniform in size than those of the endoderm. At 

 their inner ends they are narrower, and usually end in 

 slender prolongations, which bend at a right angle to the 

 main axis of the cell, and help make a sort of middle layer 

 between the endoderm and ectoderm. These prolonga- 



Tentacl 



Spermary 



Bud 



Cross section 



FIG. 180. HYDRA, LONGITUDINAL SECTION. 



tions are called muscle processes, and are the chief agents 

 in shortening and moving the body. Between the nar- 

 rowed bases of the larger ectoderm cells are smaller cells, 

 which are supposed to be sensitive, and may perhaps be 

 properly called nerve cells. 



Stinging Cells. Among the cells of the ectoderm, both 

 of the body and of the tentacles, are peculiar bodies called 



