184 



ZOOLOGY 



hydroids, the spongps/ the mussels, and the barnacles (Fig. 

 174), or else the.y have some device for clinging fast, as 

 in the case of the starfish, and many snails and crabs. At 

 the opposite physiographic extreme we find the sea penetrating 

 as a harbor deep into the mainland.- The boundary between 

 the sea and the land is in tliese cases often very uncertain 



Fig. 17.5. —An enclosed harbor or arm of the gca i.tenetrating deep into the 



niainUrnd. 



because the semiaquatic plants grow far out into the water, 

 forming the edge of a salt-marsh. Here twice a day the sea 

 bottom is ex]iosed to the air l)y the retreating tide, and terres- 

 trial animals wand(>r out upon it to feed. In this situation the 

 permanent animals are those that burrow in the mud or pene- 

 trate between the roots of marsh-grass or swim about on the 

 surface of the water (Fig. 17G). These animals feed on the 

 debris that floats on the surface of the water or else crawl 

 about on the bottom to Ijrowse on the algfe which flourish 



1 See C^haptcr XIX. ^ Fig. 175. 



