2 HlCKSON, Animal Symmetry. 



led to the evolution of the two principal types of symmetry 

 and the influence that symmetry has had in the differentia- 

 tion of those smaller discontinuous groups of animals 

 which we call species. 



In many of the higher groups of animals, such as the 

 Vertebrata, Arthropoda and Annelida, we find that the 

 body shows an almost perfect bilateral symmetry. If we 

 take such an example as a fish and divide the body 

 vertically in a plane running from the snout to the tip of 

 the tail, we divide the body into two equal parts in which 

 the organs of the one part are almost an exact reflection 

 of the organs of the other part. Moreover, this plane, 

 from snout to tail, is the only plane in which the body 

 can be divided into two equal parts with a similar arrange- 

 ment of the organs. The bilaterally symmetrical body 

 also shows an anterior end which goes first in movement, 

 and a posterior end which follows, a dorsal surface which 

 is heliotropic, and a ventral surface which is geotropic and 

 a right and left side. 



This bilateral symmetry of the animal body is usually 

 accompanied by the power of rapid movement in a definite 

 direction. All Vertebrate animals, Arthropods and Anne- 

 lida, which possess the power of strong muscular move- 

 ments in a definite direction through the air or water or 

 on the land, have a definite bilateral symmetry. But 

 this is not confined to animate things. A railway train, 

 a steamship, or a motor car show an anterior and posterior 

 end, a dorsal and ventral surface, and a right and left 

 side. They are also essentially bilaterally symmetrical. 

 It is, in fact, an essential feature of objects that move 

 rapidly and efficiently by their own power that they should 

 have this kind of symmetry. 



But although all animals that move rapidly are bi- 

 laterally symmetrical, it is also a fact that there are others 



