COELENTERATA 137 



nerve tissue, sense organs, the skeleton, a definite ali- 

 mentary canal, and definite reproductive organs. 



Distinguishing Characteristics. — The name coelen- 

 terata comes from two Greek words that mean hollow 

 intestine. It was given to the group because of the 

 prominent digestive cavity which occupies the whole of 



Fig. 76.— Hydra showing tentacles and the enlargements which give rise to the 

 sperm and egg cells. (From Hertwig.) 



the interior of the body, to the exclusion of the separate 

 and distinct body cavity which exists in most of the 

 higher forms. This common characteristic marks the 

 relationship between such apparently diverse forms as 

 the hydra, the jellyfish, and the sea anemone (Figs. 76, 

 77, 78). In the hydra there are only two layers of cells 

 and these are arranged in a double, bell-shaped wall so 



