522 RICHARD EVANS. 



yolk which is to be stored in it in the not very distant future, 



such as is now found in the ovum of both Eoperipatus and 



Peripatoides. The first explanation is the one put forward 



by Sedgwick, and seems to be the only possible one; for the 



second alters the sequence of events in such a way that the 



effect precedes the cause. The cause of the large size of all 



ova seems to l)e the presence of yolk in one form or another. 



But there is no yolk in P. cap en sis if we are to accept 



Sedgwick's account of the structure of the ovum in that 



species. From these considerations it seems fairly evident 



that it is not equally possible to explain the structure of the 



ovum of P. capensis in both ways, and that the only 



possible explanation of it is the one put forward by Mr. 



Sedgwick. It has been pointed out above that the main 



reason for assuming the ovum of the Neotropical genus, 



Peripatus^ to be primitive as compared with that of the 



New Zealand genus. Per ip at op sis, is the generally primitive 



character of the former, and Dr. Willey might justifiably say 



that there is no such difficulty in explaining the structure of 



the ovum of Peripatus and Paraperipatus as is raised 



by the explanation of the case of Peripatopsis. But 



now that the Malay forms have been discovei'ed, forms 



which in all respects are as primitive, and in some respects 



more primitive, than the Neotropical genus; — e.g., in the 



position of the renal opening of the fourth and fifth pairs of 



legs; the presence in the species Horsti of a well-developed 



renal organ in the last pair of legs, and in the species 



Weldoni of a vestigial one, — a new difficulty crops up, for we 



have two groups, to which the dignity of genera has been 



accorded in the present memoir, — which are closely related in 



all their external and internal characters except in those of 



the ovum, ovary, and mode of development. In the genus 



Peripatus the ovary is small and compact, and the ova 



arise endogenously and are devoid of yolk, but in the 



genus Eoperipatus the ovary is large and spreads out, and 



the ova arise exogenously and are full of yolk. It becomes 



necessary to decide which of these conditions is the more 



