136 



PHYSIOLOGY 



assumed the functions of digestion and absorption of 

 food substances. For some time probably animals re- 

 mained in this primitive stage; there is still in exist- 

 ence a large group which has never passed beyond it. 

 Later a third layer, the mesoderm, developed between 

 the ectoderm and the entoderm, and the three primary 

 layers were established. All higher animals pass 

 through the layer stage in their development. From 

 these layers are developed their highly organized tis- 

 sues; from the ectoderm the protective organs, — skin 

 and its appendages, nerves and sense organs; from the 



entoderm everything which 

 has to do with alimenta- 

 tion, — digestive tract, di- 

 gestive glands, lungs ; 

 from the mesoderm, every- 

 thing else, — bone, muscles, 

 blood, connective tissue, 

 many glands. 



Tissues of the Coelen- 

 terata. — In the coelente- 

 rata all the functions of the 

 body are performed by the 

 two primary layers (Fig. 

 75). The third layer is 

 rudimentary. This group 

 of animals is particularly 

 interesting because it is 

 transitional. Within it 

 most of the tissues found in higher organisms are es- 

 tablished. We have the beginning of muscle tissue, 



8 ek 



Fig. 75.— Section of wall of hydra, en, 

 nettle cells ; ek, ectoderm ; s, sup- 

 porting layer ; en, entoderm. (From 

 Hertwig, after Schulze.) 



