140 



PHYSIOLOGY 



to the outside through which food enters and waste is 

 ejected. 



Guarding this opening is a circlet of tentacles, highly 

 developed, contractile organs. In the tentacles are de- 



Fig. 79.— Diagram of sections of J, hydroid • B, jellyfish ; C, sea anemone. (After 

 Hertwig.) 



veloped nettle cells, each of which has a little tube 

 coiled in it which can be thrown out when the cell is 

 stimulated. The tubes set free an irritating poison 

 which paralyzes smaller animals and serves not only. as 

 a ..means of ..defense -but aids, in the capture of food. 



