18 



GENERAL ZOOLOGY 



lateral. Man may serve as an example of a bilaterally 

 symmetrical animal. 



5. Body Sac-like or a Tube Within a Tube, i.e., Accelomate 

 or Ccelomate (Fig. 16). Some animals, like the fresh-water 



FIG. 16. A Hydra and the anterior end of an earthworm. Both have been 

 split lengthwise so that the inside of the body is exposed. The Hydra has only 

 one cavity, the enteron, or digestive cavity; the earthworm has another cavity, 

 the ccelom, between the digestive tube and the body wall. 



FIG. 17. Showing endo- and exoskeletons. The bones in a man's leg are 

 surrounded by muscles; the skeleton of a grasshopper's leg consists of tubes 

 with muscles inside. 



hydra, have only one cavity within the body and the out- 

 side wall is solid. There is only one opening into the cavity, 

 the mouth. Other animals, like the earthworm, have a 



