150 THE SHAPES AND SIZES OF ANIMALS 



ence of size on structure. Of these one of the 

 most interesting is that afforded by the genera 

 Aplysia, Doris, Eolis, and Pontolimax, all four of 

 which belong to the group of Opisthobranchiate 

 Gastropods, or sea slugs. Aplysia, the sea hare, 

 is an animal of some size, four or five inches in 

 length, and has a very well-developed gill covered 

 over and protected by a thin uncalcified shield-like 

 shell. Doris is of smaller size, has no shell, and 

 has its gill processes less developed than in Aplysia, 

 but still grouped together in a tuft on the hinder 

 part of its back. In Eolis, there are a number of 

 elongated papillae covering the back of the animal, 

 some of which, at any rate, act as respiratory 

 organs. Pontolimax, finally, is a very small slug- 

 like animal only a twelfth of an inch in length, and 

 having a perfectly smooth dorsal surface devoid of 

 respiratory processes of any kind. 



Herbert Spencer has pointed out very clearly 

 how in two animals of the same zoological type, 

 but of markedly different size, the smaller one may 

 be able to do without respiratory organs, while to 

 the larger one, merely in consequence of its larger 

 size, such organs are absolutely essential. Suppose 

 you have two animals of identical shape, but of 

 different size : for the sake of simplicity let us 

 suppose the animals to be spherical, and let the 

 diameter of the smaller animal be one inch, that of 

 the larger two inches. The extent of surface in 

 the two will be proportionate to the squares of their 

 linear dimensions i.e. t in this case, the larger animal 

 will have a surface four times as extensive as the 



