Third Section. 



Vermes. 



General Review* 



§ 99. 



In this division a large number of animal forms, whicli are more 

 or less allied to one another, are put together ; transverse axes are 

 differentiated, whilst the longitudinal axis of the body is elabo- 

 rated. In consequence of this an anterior and a posterior end can 

 be made out, in addition to a dorsal and ventral surface. They 

 differ markedly from the Coelenterata in having two antimeres. 

 The body is, or is not, divided into metameres ; in the more simple 

 forms the metameres are simple in character, in the higher divisions 

 they undergo differentiation. 



It is not certain whether this division does or does not represent 

 a single phylum. The existence of a large number of small groups, 

 represented merely by single forms, shows a considerable amount of 

 divergence within the division ; and this is still further exhibited by 

 the fact that almost all the higher animal phyla can be brought into 

 more or less close connection with forms of Vermes. 



I arrange the various divisions of the Vermes in the following 

 order. They might be considerably increased by the introduction 

 of a large number of isolated genera ; but a complete classification 

 of such a kind is not part of our purpose here. 



I. Platyhelminthes. 

 Turbellaria. 



Rhabdocoela. 



Monocelis, Vortex, Mesostomum, Prostomum. 

 Dendroooela. 



Planaria, Leptoplana, 



