100 



ANIMAL BIOLOGY 



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 (ecto- 

 derm and endoderm) have not become differentiated into secondary 



Ectoderm 

 Mesogloea 



Endoderm 

 Enteric cavity 



Fig. 58. — Hydra. Transverse section highly magnified. 



specialized tissues (muscular tissue, nerve tissue, etc.) for one 

 function or another — the simple life processes of the animal are 

 adequately provided for by the specialization of isolated cells or 

 small cell groups within ectoderm and endoderm. (Fig. 58.) 



B. Earthworm 



The bodies of all animals above the Coelenterates are built of 

 three primary layers, which, as development of the individual 

 proceeds, give rise to the secondary tissues and thereby form a 

 relatively complex body. This third primary layer (tissue), the 

 mesoderm, typically is developed, as we have described earlier, 

 from the endoderm and comes to occupy the position held by the 

 mesogloea of Hydra ; that is, between the ectoderm and the endo- 

 derm. 



The development of the mesoderm is the key to the advance in 

 body organization of higher animals, because it makes possible 

 a radical change in plan that involves the establishment of a body 



