THE ANIMAL BODY 



119 



in which but two primitive tissues exist; such specializations 

 as are necessary for the performance of the essential life func- 

 tions being confined to the more or less isolated cells of these 

 layers. The majority of the cells of the endoderm which 



FIG. 64. Hydra. Longitudinal section, magnified. 1, mouth; 2, tentacles; 

 3, early stage in budding; 4< older bud; 5, ectoderm; 6, endoderm; 7, enteric 

 cavity; 8, testis; 9, ovary. (From Linville and Kelly, after Parker.) 



line the ENTERIC CAVITY are concerned with the digestion of 

 solid food taken in through the mouth, while those of the 

 ectoderm are variously modified for protection, and the other 

 relations of the individual to its surroundings, as well as for 

 reproduction. 



In short, in the organization of Hydra the primary tissues 

 (ectoderm and endoderm) have not become differentiated 



