50 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 97 



cussions of relationships between the Annehda, the Onychophora, 

 and the Arthropoda. The onychophoran nephridia, however, are 

 developed as simple diverticula of the ventral walls of the coelomic 

 sacs, which connect with short ectodermal ingrowths of the same 

 segments situated mesad of the leg bases, and the nephridial organs 

 of the arthropods most probably have had the same genesis as the 

 onychophoran organs. Hence, it is possible that the coelomic exits 

 have had an independent origin in the higher Annelida on the one 

 hand, and in the common ancestors of the Onychophora and Arthrop- 

 oda on the other. 



V. THE ONYCHOPHORA 



Somewhere from a generalized annelid stock there must have 

 branched off in remote pre-Cambrian time the ancestors of the group 

 of animals that includes the modern Onychophora (fig. 21 A), the 

 Cambrian Aysheaia (B), and the pre-Cambrian Xenusion (C). The 

 primitive onychophorons undoubtedly were segmented, wormlike 

 creatures, in which coelomic sacs and the basic features of the annelid 

 muscular and nervous systems had long been established, and in 

 which the body had been lengthened by the addition of a series of 

 reproductive somites generated from the posterior zone of growth. 

 A distinctive feature of the Protonychophora, however, was the pos- 

 session of movable locomotor appendages having the form of small 

 lobelike outgrowths of the body wall along the lateroventral lines of 

 the segments. The ancestors of the lobopod Onychophora, and the 

 ancestors of the chaetopod Annelida, therefore, probably constituted 

 two divergent branches from a generalized annelid stock. The primi- 

 tive chaetopods were creeping worms that progressed by the usual 

 vermiform movements of the body, produced by the body musculature 

 with the aid of integumental chaetae. The primitive onychophorons 

 became distinguished as walking worms, a character well expressed 

 in the name Peripatus (Guilding, 1826) given to the first-described 

 modern form. The walking habit led to the adaptation of the modern 

 Onychophora to life on land, but the older forms, such as Aysheaia 

 and Xenusion, may have been inhabitants of the ocean. 



A typical onychophoron is a slender wormlike creature with a pair 

 of tentacular antennae at the anterior end of the trunk, and a double 

 row of short, conical, lateroventral legs along the length of the body 

 (fig. 21 A). The trunk is cylindrical or somewhat depressed, blunt 

 anteriorly, and tapering posteriorly. The rough integument is closely 

 ringed, but there is no external sign of segmentation except for the 

 series of appendages. The animal has no distinct head ; the anterior 



