130 ANIMALS 



differentiation results in a repetition of similar parts and these 

 parts must either be arranged radially or serially. But the 

 serial arrangement results in an elongated body and this is 

 better adapted for free locomotion. Consequently the body 

 of the typical animal is elongated in the axis of locomotion. 



311. Cephalization. — Since locomotion is generally in a 

 horizontal direction the elongation of the body is horizontal. 

 But the two poles of this body are not alike, because the prin- 

 ciple of division of labor and efficiency would make locomotion 

 in one of the two directions become the principal direction of 

 locomotion. The animal usually moves with the same end 

 forward and this end, which is called anterior, is very different 

 from the opposite or posterior end. The difference is chiefly 

 due to the development of special sense organs at the anterior 

 end, because this end comes more positively into relation with 

 the forces which afi'ect the senses. The development of the spe- 

 cial sense organs further carries with it the special development 

 of the central nervous system of that region; that is, the devel- 

 opment of a brain. The locomotion of the animal has to do 

 largely with obtaining food and this probably determines that 

 the anterior end is located near the mouth. Then the develop- 

 ment of organs for ingestion and comminuting food, and the 

 sense organs connected with this function still further dift'eren- 

 tiate the anterior end from the posterior. The development of 

 all these organs at the anterior end of the animal forming a com- 

 plex of organs called the head is called cephalization. The 

 posterior end of the body is sometimes developed into an organ 

 of propulsion or otherwise speciaKzed, but never to the degree to 

 which the more positive conditions bring the development of 

 the anterior end. In this way are determined the elongation 

 of the animal with its principal axis horizontal, and the differ- 

 entiation of the two poles into anterior and posterior. 



312. Dorsal and Ventral. — For animals which pass from one 

 medium to another, as from water to dry land or from the latter 



