AFFINITIES. 551 



The genus Peripatus was established in 1826 by Guilding, who 

 first obtained specimens of it from St. Vincent in the Antilles. 

 He regarded it as a Mollusc, being no doubt deceived by the 

 slug-like appearance given by the antennae. Specimens were 

 subsequently obtained from other parts of the neotropical region 

 and from South Africa and Australia, and the animal was vari- 

 ously assigned by the zoologists of the day to the Annelida and 

 Myriapoda. Its true place in the system, as a primitive member 

 of the group Arthropoda, was first established in 1874 by Moseley, 

 who discovered the tracheae. 



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, 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, has had to 

 be created for its sole occupancy. This unlikeness to other Ar- 

 thropoda is mainly due to the annelidan affinities which it 

 presents, but in part to the presence of the following peculiar 

 features : (1) The number and diffusion of the tracheal aper- 

 tures ; (2) the restriction 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 dermo- 

 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 determination 

 constitutes one of the most interesting points of the later work 

 on the genus. The annelidan features are : (1) The paired 



