FOIiM OF THE BODY OF VERMES. 



129 



axes is not expressed, so that similar characters to tliose which 

 obtain in the Coelenterata 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, which 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 body. 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. 



Another phenomenon which is first seen among Vermes is the 

 segmentation of the body. Already in the 

 Rotatoria the hinder portion of the body 1. 2. 



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- 

 lafsi 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 Tseniadae 

 a very large series of these generative rarhynchns) ; asexu: 



? i -i • ,i i • i i r (nurse). 2. The same iu the 



organs are developed in the hinder end of > oint _ forming stage (strobila)> 

 the body, and each corresponding area in which the last joints (pro. 

 forms a joint, which is gradually marked glottids) are breaking off, one 

 off on the outer surface, and has the rela- by°ne(afterP.J.vanBeneden). 

 tions of a metamere to the other joints 



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

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



Fit?. 51. 



Taenia (Tot- 

 al form 



