60 COMPAEATIVE ANATOMY. 



tiation of its surfaces^ is retained in the case of tlie transverse axis. 

 This, the second form, which can be derived from the Gastrula, and 

 is ordinarily known as that of bilateral symmetry, first appears in 

 the Vermes, and prevails in all the divisions above them. 



When the secondary axes of the body retain their primitive 

 indifferent character, we may imagine that there are as many similar 

 pieces in the architectural composition of the body as there are 

 possible secondary axes. But when the secondary axes become 

 differentiated, the divisions of the body take on a definite numerical 

 relation. They are known as antimeres. If two secondary axes 

 become differentiated, and are like in character, we have four 

 antimeres, for we can divide the body into four similar parts, along 

 these secondary axes. But when two unlike secondary axes are 

 differentiated the body is only made up of two antimeres ; two 

 halves of the body, distinguished as right and left, are the parts 

 corresponding to one another. In this way the eudipleural funda- 

 mental form is developed. 



§ 50. 



The differentiation which marks off the oral from the aboral pole, 

 and which has been already mentioned, gives a higher significance to 

 the former region of the body. This differentiation asserts itself in 

 other forms, as in the primaiy radiate form, and in very various ways. 

 It is not only the presence of the mouth, which favours the 

 differentiation of organs around it, as organs for aiding in the 

 prehension or ingestion of food, but the greater significance of the 

 anterior end of the body in locomotion is also a cause of differen- 

 tiation. This part takes the initiative. It has to show the way to 

 the rest of the body, and often indeed, to lead it ; it meets with a 

 thousand foreign objects, which it has to examine, to follow, or to 

 avoid. It is therefore exposed to external influences other than 

 those which act on the opposite end of the body. The dignity of 

 the relations of this region explains how it is that the mouth is not 

 by any means always at the anterior end of the body, and that it 

 much more frequently is placed close to or even altogether on the 

 ventral surface, without the anterior end of the body being less highly 

 developed. The high specialization of the anterior region is caused 

 principally by the development of various kinds of sensory organs, 

 and therefore of organs which put the organism into relation with the 

 outer world ; the region moreover often has various organs of defence 

 connected with it, and with it is closely connected the development 

 of the central nervous system. The whole region thus gets a higher 

 value in comparison with the general organism, for it shelters and 

 carries the organs which elevate and rule the latter. This anterior 

 region of the body is therefore called the head, or chief portion. 

 The differentiation of a head seems to depend primarily on the 

 position of the mouth. This du-ects the course of movement, and 

 to this, as a secondary cause, the anterior part of the body owes its 



