148 ANIMALS 



protective, but in special cases it may serve a variety of other 

 functions. Sometimes it serves to cement together the parti- 

 cles of earth to form a tube in which the animal lives. Some- 



FIG. 72. A parchment-like tube constructed by a marine annelid, Chsetop- 

 teris. The tube is buried in the mud, except an inch or two of each end which 

 project above the surface. Both ends are open to permit a current of water to 

 pass through. X 2/3. 



times it is used in locomotion, as in case of snails. The skin 

 of fishes is richly supplied with slime glands. Hydra and 

 many other animals attach themselves temporarily or per- 



