136 



ANIMALS 



revolves 90° on its principal axis, its plane of symmetry remains 

 fixed. That is, of course, what one should expect following the 

 general principles already laid down; for since there is no change 

 in the external forces which cause symmetry in the organism 

 there should be no change in the plane of symmetry following 

 the revolution of the animal on its axis. 



Fig. 66. — The flounder, Pseudopleuronectes Americanus, showing approximate 

 dorso-ventral symmetry. Note that both eyes are on the right side of the head. 

 (From Hegner, after Goode.) 



323. Size and Differentiation. — Animals vary greatly in size 

 and complexity of structure, from the microscopic protozoa to 

 the gigantic mammals, and it is of interest to note that, in a 

 general way, size and complexity vary together. Superiority 

 in size is of itself an advantage, especially where there is a 

 contest between individuals. But more important is the advan- 

 tage derived from complexity, which permits of differentiation, 

 division of labor and consequent efficiency. Considerable 

 differentiation may be found between the parts of the same cell, 

 as in the protozoa, but when the body is composed of many 

 cells the differentiation may be vastly greater, both in regard to 

 the number of kinds of differentiation and the degree to which 

 it is carried. Thus the functions of contraction, irritabihty, 



