PERIPATUS 



There can be no doubt that Peripatus is an Arthropod, for it 

 possesses the following features, all characteristic of that group, 

 and all of first-class morphological importance: (1) The presence 

 of appendages modified as jaws ; (2) the presence of paired lateral 

 ostia perforating the wall of the heart and putting its cavity in 

 communication with the pericardium; (3) the presence of a vas- 

 cular body cavity and pericardium (haemocoelic body cavity) ; 

 (4) absence of a perivisceral section of the coelom. Finally, the 

 tracheae, though not characteristic of all the classes of the 

 Arthropoda, are found nowhere outside that group, and constitute 

 a very important additional reason for uniting Peripatus with it. 



Peripatus, though indubitably an Arthropod, differs in such 

 important respects from all the old-established Arthropod classes, 

 that a special class, equivalent in rank to the others, and called 

 Prototracheata, has had to be created for its sole occupancy. 

 This unlikeness to other Arthropoda is mainly due to the Anne- 

 lidan affinities which it presents, but in part to the presence of 

 the following peculiar features: (1) 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 tex- 

 ture of the skin ; and (5) the simplicity and similarity of all the 

 segments of the body behind the head. 



The Annelidaii affinities are superficially indicated in so 

 marked a manner by the thinness of the cuticle, the dermo- 

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

 stated, many of the earlier zoologists who examined Peri)><ilnx 

 placed it amongst 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: (1) The paired 

 nephridia in every segment of the body behind the first two 

 (Saenger, Balfour x ) ; (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 con- 

 junction with the Annelidaii 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 



1 F. M. Balfour, "The Anatomy and Development of Peripatus caj)ensis," edited 

 by Professor H. N. Moseley and A. Sedgwick, Quart. Journ. Mic. Sci. xxiii. pp. 

 213-259, pis. xiii.-xx. 1883. 



