150 TEE AMERICAN NATURALIST [Vol. LYI 



waves and tides sweep, he may be surprised to find an 

 abundant fauna in the 'tween-tide " zone. The moving 

 water, teeming with microscopic organisms, brings an 

 abundance of food to those animals that are able to stand 

 the beating of the waves and the alternate submergence 

 and exposure due to the ebb and flow of the tides. A 

 rocky wall along the sea shore is no place for weaklings. 

 One minute the blistering sun bakes the exposed animals ; 

 the next, the rising tide has covered them with cold water. 

 The waves beat ceaselessly. The changing seasons bring 

 ice and torrid heat. How are the animals on these rocky 

 shores responding to the environment? Here one finds 

 a variety of hardy species which, though not closely re- 

 lated genetically, have many characteristics in common. 

 There are sponges, anemones, hydroid colonies, barnacles, 

 mussels, snails, small crustaceans, and a few scavenger 

 crabs. These animals for the most part obtain their food 

 by net fishing or by straining water through their bodies. 

 They are mostly attached firmly to the rocks, and thus 

 withstand the violent movements of the food-laden water. 

 The barnacles, sponges, and hydroids are grown fast ; the 

 anemones and snails have sucking discs that enable them 

 to adhere fiimly; the crustaceans have claws for attach- 

 ment and hard armor covering their bodies. Some of 

 these animals are small and can easily hide in crevices ; 

 some of those of larger size, like crabs, are able to migrate 

 to other habitats during violent storms. If an animal is 

 attacked, it is advantageous for it to be able to receive 

 stimuli with facility from all directions of the compass, 

 and, as would be expected, many of the animals on rock 

 beaches are radially symmetrical. Radial symmetry has 

 marked advantages for sessile animals, Init puts a 

 weighty limitation on psychic development. An animal 

 that is able to perceive stimuli equally well, through 

 equally efficient sets of sense organs that are symmetrical- 

 ly disposed about a central axis, is never able to develop 

 its power of paying attention to any coMsi(l(M-a])le degree. 

 Its simple mind, if such nii aniiiial may be said to have 



