2O2 MANUAL OF ZOOLOGY. 



" hanging like a globe in the centre of the body-cavity," but not 

 communicating with the body-cavity. 



AFFINITIES OF ROTIFERA. In their external appearance the 

 Rotifera approximate closely to the Infusoria, but the organisa- 

 tion of the former presents a very striking advance when com- 

 pared with that of the latter. Thus, in the Infusoria there is 

 no differentiated body-cavity, bounded by distinct walls, and 

 the alimentary canal is imperfect, the digestive sac simply 

 opening inferiorly into the diffluent sarcode of the centre of 

 the body. Further, there are no traces of a nervous system, 

 and the contractile vesicles, if looked upon as representing the 

 water-vascular system, are a very rudimentary form of this 

 apparatus. In the Rotifera, on the other hand, the alimentary 

 canal forms a complete tube, having an oral and an anal aper- 

 ture, and not communicating with the surrounding perivisceral 

 cavity; and there is a well-developed nervous system, and a 

 highly complex water-vascular system. A real affinity is found 

 to subsist, however, between the Rotifera and the Planarida ; 

 both possessing external cilia, a nervous system, and a well- 

 developed water- vascular apparatus, the characters of which are 

 not dissimilar in the two groups. In the Planarida, how- 

 ever, the sexes are united in the same individual, and there is 

 no anal aperture ; whereas in the Rotifera the sexes are dis- 

 tinct, and there is a distinct anus. To the true Arthropoda^ as 

 already pointed out, the Rotifera shows some points of affinity, 

 but these are hardly sufficiently numerous or decided to war- 

 rant the removal of the group from the Annuloida to the 

 Annulosa. 



