The large number and the uniformity of the trunk somites and their appendages, and the structure of the nervous system and of the heart in A pus, are Annelidan characters which can hardly be without significance.
This unlikeness to other Arthropoda is mainly due to the Annelidan affinities which it presents, but in part to the presence of the following peculiar features: (I) the number and diffusion of the tracheal apertures; (2) the restriction of the jaws to a single pair; (3) the disposition of the generative organs; (4) the texture of the skin; and (,) the simplicity and similarity of all the segments of the body behind the head.
The Annelidan affinities are superficially indicated in so marked a manner by the thinness of the cuticle, the dermomuscular body-wall, the hollow appendages, that, as already stated, many of the earlier zoologists who examined Peripatus placed it among the segmented worms; and the discovery that there is some solid morphological basis for this determination constitutes one of the most interesting points of the recent work on the genus.
The Annelidan features are: (I) the paired nephridia in every segment of the body behind the first two (Saenger, Balfour); (2) the presence of cilia in the generative tracts (Gaffron).
It is true that neither of these features is absolutely distinctive of the Annelida, but when taken in conjunction with the Annelidan disposition of the chief systems of organs, viz.