552 



CLASS II. OKYCHOPHORA. 



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 conjunc- 

 tion with the annelidan disposition of the chief systems of organs, 

 viz. the central nervous system, and the main vascular trunk 

 or heart, they 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 organization most closely related. It 

 stands absolutely alone as a kind of half-way animal between the 

 Annelida and Arthropoda. There is very little gradation of 



FIG. 331. Peripatus ca/jensis, drawn from life. Life size. (After Sedgwiek.) 



structure within the genus ; the species are limited in number, 

 and in all of them the peculiar features above mentioned are 

 equally sharply marked. 



Peripatus is a segmented animal, the segments being marked 

 by the appendages. The ridges of the skin are considerably 

 more numerous than the true segments. The number of seg- 

 ments is usually variable in the same species, but it is always 

 complete at birth being definitely fixed at an early stage of 

 development. The anterior part of the body may be called 

 the head, though it is not sharply marked off from the rest of 

 the body (Fig. 331). The head carries three pairs of appendages, 

 a pair of simple eyes, and a ventrally placed mouth. The body 

 is elongated and vermiform ; it bears a number of paired ap- 

 pendages, each terminating in a pair of claws, and, except for 

 the fact that the last pair or last two pairs are occasionally 

 somewhat reduced, all closely alike. The number varies in the 



