In their external appearance the Rotifera approximate closely to the Infusoria, but the organisation of the former presents a very striking advance when compared 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 aperture, 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, 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 Scolecida to beside the higher Annulosa.