rOEM OF THE BODY OF VEE^IES. 



129 



axes is not expressed, so tliat similar cliaracters to those whicli 

 obtaiu in tlie Ca3lentcrata can be made out, yet I do not regard this 

 condition as one that has been inherited by the Cestoda, for they can 

 only be derived from forms, which like the rest of the Platyhelminthes, 

 possessed the original eudipleural form. Their condition, which 

 depends on the equal development of the secondary axes, is at once 

 explained by their loss of the power of locomotion, and by the 

 attachment of the body by a point, Avhich corresponds to one pole 

 of the primary axis. 



A head, which has a mouth placed as a rule somewhere on its 

 ventral surface, can generally be distinguished at the oral pole. In 

 most Platyhelminthes the mouth is some distance from the head ; in 

 the Turbellaria, indeed, it is generally some way back on the ventral 

 surface of the hodj. The aboral end of the body carries the anus ; 

 this, when present, has ordinarily a dorsal position. 



In the fixed Vermes the form of the body undergoes considerable 

 modifications. It is greatly influenced by the development of a 

 covering, as in the Bryozoa. The aboral end of the body, by which 

 the animal is attached, can no longer carry the anus, which is 

 accordingly placed nearer to the anterior portion of the body, which 

 is not enclosed by the cell. 



§ 101. 



1. 



2. 



Another phfenomenon which is first seen among Vermes is the 

 segmentation of the body. Already in the 

 Rotatoria the hinder portion of the body 

 is adapted to locomotion by being broken 

 up into a number of segments. In this we 

 see an indication of a condition which 

 becomes very important in the higher divi- 

 sions. In the Cestoda it is further de- 

 veloped. A differentiation is occasioned 

 by the growth of the body in the direction 

 of its primary axis. The anterior and pos- 

 terior parts of the body no longer enclose 

 the same organs. Thus in the Caryophyl- 

 laei the hinder portion of the body alone 

 contains the generative organs. In Ligula 

 this hinder portion of the body is consider- 

 ably developed by the great repetition of 

 the generative organs. In the Tfeniada3 

 a very large series of these generative 

 organs are developed in the hinder end of 

 the body, and each corresponding ai'ea 

 forms a joint, which is gradually marked 

 off on the outer surface, and has the rela- 

 tions of a metamere to the other joints 



(Fig. 51). In this way the Ttenia-chain is formed, the last meta- 

 meres of which (the so-called proglottids) break off at a certain 



K 



Fig. 51. 1. Tjcnia (Tet- 

 rarhyuchus) ; asexual form 

 (mirse). 2. The same in the 

 joint-forming stage (strobila), 

 in which the last joints (pro- 

 glottids) are breaking oil, one 

 tayone(afterP.J.vanBeneden). 



