158 MANUAL OF ZOOLOGY. 



ture, and not communicating with the surrounding perivis- 

 ceral 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 

 Planaridci', 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 Plana- 

 rida, however, the sexes are united in the same individual, 

 and there is no anal aperture ; whereas in the Rotifera, the 

 sexes are distinct, and there is a distinct anus. To the true 

 Arthropoda, as already pointed out, the Rotifera show some 

 points of affinity, but these are hardly sufficiently numerous or 

 decided to warrant the removal of the group from the Annu- 

 loida to the Annulosa. 



