130 THE SHAPES AND SIZES OF ANIMALS 



symmetry, seen in its most typical form in the 

 gastrula, is confined to aquatic forms. The reason 

 for this is the same as in the case of the still more 

 primitive spherical form i.e., that it is only in the 

 case of animals, whether young or adult, which 

 live immersed in fluid, that the relations between 

 the animal and the surrounding medium are such 

 as to allow of the animal having identical relations 

 to the environment, whichever part of its circum- 

 ference happens to be uppermost or undermost. It 

 is also a matter of common observation that radi- 

 ally symmetrical animals are not merely all aquatic, 

 but are almost entirely marine. The reason for 

 the paucity of radially symmetrical forms in the 

 fresh-water fauna appears to be that they are weak 

 swimmers, depending for locomotion on the action 

 of cilia when the animals are of small size, and 

 having no special locomotive organs when of larger 

 dimensions. Fresh-water animals, at any rate 

 such as dwell in rivers and streams, have to be 

 able to hold their own against the currents in which 

 they live ; nay more, they must not merely be 

 able to hold their own but also to make their way 

 up stream as well as down, or else in the long 

 run they will be carried slowly but steadily lower 

 and lower down the river, until ultimately they 

 become swept out to sea. For this reason weakly 

 swimming animals cannot, unless under excep- 

 tional circumstances, establish themselves in fresh 

 water. 



While the typically radial animal is a free swim- 

 ming form, there are a large number that in the 



