154 THE GENUS PERIPATUS. 



of the jaws to a single pair, (3) the disposition of the generative 

 organs, (4) the texture of the skin, and (5) 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 derrao- 

 muscular body wall, the hollow appendages, that, as already 

 stated, many of the earlier zoologists who examined Peripatus 

 placed it amongst the segmented worms ; and the discovery 

 that there is some solid morphological basis for this determin- 

 ation constitutes one of the most interesting points of the 

 recent work on the genus. The Annelidan features are : (1) 

 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 are absolutely distinctive of the Annelida, but when 

 taken in conjunction with the Annelid disposition of the chief 

 systems of organs, viz. the central nervous system, and the 

 main vascular trunk or heart, may be considered as indicating 

 affinities in that direction. Peripatus, therefore, is zoologically 

 of extreme interest from the fact that, though in the main 

 Arthropodan, it possesses features which are possessed by no 

 other Arthropod, and which connect it to the group to which 

 the Arthropoda are in the general plan of their organisation 

 most closely related. It must, therefore, according to our 

 present lights, be regarded as a very primitive form ; and this 

 view of it is borne out by its extreme isolation at the present day. 

 Peripatus stands absolutely alone as a kind of half-way 

 animal between the Arthropoda and Annelida. There is no 

 gradation of structure within the genus ; the species are very 

 limited in number, and in all of them the peculiar features 

 above mentioned are equally sharply marked. 



We may, therefore, with some justice, regard Peripatus as 

 an animal which has persisted for a long time, with but little 

 structural modifications ; as the representative of an ancient 

 group, once widely diffused,^ and probably rich in species and 



' Tliat the class had once a world-wide diffusion is indicated by the wide 

 and discontinuous distribution of the living species. 



