INVERTEBRATE BOTTOM FAUNA 471 



have extremely bent legs and claws, the naked snails have their 

 flexible foot-discs and the planarians their rhabdites, so that 

 these creatures furnish excellent examples of adaptability to 

 external conditions. A bodily structure of this kind is necessary 

 for these forms, or when exposed to the action of the waves 

 or currents they would run the risk of being torn from the 

 objects to which they cling. 



The marine algse are known to be rather particular about 

 the localities they select. Some species grow high up on the 



Fig. 334. 

 Asterias glacialis, L. (After Ludwig. ) , 



rocks so as to be covered only at high tide, while others choose 

 the lowest limit of ebb-tide ; some prefer sunlight, while others 

 thrive only away from it ; some grow best amidst the waves and 

 breakers, while others need sheltered places. This is, to some 

 extent, true also of the animal forms of the upper littoral zone, 

 many of which prefer the open parts of the coast, while others 

 live in sheltered localities, and others again where the currents 

 are strong. The three bryozoans Alcyonidiuni, Flustrella, and 

 Bowerbankia, for instance, seem to prefer shelter and a good 

 current, whereas Membranipora pilosa flourishes best in the 

 laminaria belt, in exposed places where Laminaria hyperborea 



