■< — Epidermis >4|-< — 



Tissue arrangement 

 (after Storer and Usinger) 



Muscle-like cells of Hydra 

 (after Schech+er) 



Nerve-like cells In the sea anemone 

 (after Schechter) 



Fig. 18. Cell types found In coelen+erates. 



2. The body is essentially in the shape of a sack. That is, only one 

 opening exists, and this serves as both mouth and anus. The internal lining 

 of the sack ( gastrodermis ) serves to digest the food carried in through 

 the mouth, but there is no suggestion of a digestive organ system. 



3. There is no blood or circulatory system, no excretory or respira- 

 tory organs. 



4. There is a diffuse network of nerve cells, but no central nervous 

 system and only rudimentary sense organelles. 



The adult coelenterates occur in two forms. One is the polyp, which 

 has a tubular body with a closed end that is attached to the substratum. 

 There it grows singly or in colonies. The second is the medusa, a free 

 swimming, gelatinous body with a shape like an umbrella; its mouth 

 hangs down from the concave undersurface. Figure 19 shows examples of 

 these. They are variants of the same basic sack-like structure. In the polyp, 



Fig. 19. Body plans of adult coelenterates. 

 Tentacle 



