64 



CCELENTERATES 



Figure 61. — Diagram of Body 

 of Hydra. 



gloios, glutinous substance) ; 

 and the inner layer, endo- 

 derm. 



Each layer does some par- 

 ticular work for which the 

 others are not fitted. For 

 example, the outer layer 

 contains cells which are 

 especially sensitive to 

 stimuli and many modified 

 muscle cells that enable the 

 animal to move about. The 

 inner layer contains cells 

 provided with flagella which 

 catch the food particles for 

 the inner cells to digest. 

 The muscular action of the 

 outer layer moves the entire 

 animal. The sensitive cells enable the animal to recog- 

 nize its prey. The food digested by the inner layer is 

 used by all the cells of the body. Thus we see an 

 advance in the division of labor over that shown in the 

 sponge. We shall observe a still greater increase in 

 division of labor as we 

 study higher animals. 



Tentacles are hollow, 

 finger-like branches con- 

 nected with the body 

 cavity. They are pro- 

 vided with stinging cells 

 which help the hydra to 

 capture living water fleas, 

 and the like. These 

 stinging cells have darts 



Figure 62. — Microphotograph of 

 Body Wall of Hydra. 



