590 THE AMERICAN NATURALIST. [Vol. XXXVII. 



digestive cavity consists of a simple tube traversing the long 

 axis of the body, but dilated in the region of the hydranth. The 

 function of digestion is not limited to any particular region of 

 this enteric cavity, but is carried on to a greater or less extent 

 in all its parts. In Corymorpha, however, the enteric cavity 



does not remain a single tube, but becomes differentiated into 

 secondary cavities having the character of pouches and canals. 

 From the histological character of these extensions of the primi 

 tive digestive cavity, we see that they are not only morphological 

 differentiations of the latter, but that they also correspond to 

 distinct physiological differences. As stated above, the gland 



from the coenosarcal canals ; that is, the physiological activity of 

 the digestive cavity is not shared by the canals which arise from 

 it, and the function of digestion has become localized. It would 

 seem to be strongly probable that we have here a specific case 

 of localized digestion and the subsequent distribution of its prod- 

 ucts by means of the coenosarcal canals. The central primary 

 space together with the accessory spaces constitutes what might 

 be termed a gastro-vascular system, the ctenosarcal canals under- 

 taking the function of a circulatory system. The gastric sys- 



Hydra and of the simpler hydroid polypes by this exhibition of 

 a higher differentiation. 



(,/.) Tentacles.— The ectoderm of the tentacles consists of short 

 columnar cells with large distinct nuclei. Scattered among 



