THE PHYLA ARTHROPODA AND ONYCHOPHORA 



The Phylum Onychophora 



The onychophores ("claw bearers"), sometimes considered a class of the 

 phvlum Arthropoda, are a small but interesting group of animals represented 

 in the modern fauna by only about a dozen genera. Of these, the genus 

 Peripaius is best known. This "walking worm" is a predaceous animal, 

 feeding upon small insects; it is restricted to moist terrestrial habitats in 

 subtropical regions. Peripaius presents a curious mixture of characteristics, 

 some reminiscent of annelids and others resembling those of arthropods. The 

 body is worm-like, with numerous pairs of appendages arising at intervals 

 (Fig. 15.43). Unlike those of arthropods, these appendages are not jointed 

 but are fleshy, lobe-like outgrowths of the body wall containing groups of 

 muscles and bearing hooked claws at their distal ends. The head consists of 

 a simple cephalic lobe bearing antennae, simple eyes, and a mandibulate 

 mouth. The body of the adult is not segmented, although many structures 

 lie in what appears to be a segmental arrangement. In embryonic develop- 

 ment a series of paired coelomic pouches form, but the body cavity of the 

 adult is a hemocoel; remnants of the coelom are found only in the cavities 

 of the gonads and of the excretory organs. These organs are a series of paired 

 nephridia, closed internally by reduced coelomic end sacs and opening 

 externally at pores on the bases of the legs. The nephridial tubules are 

 ciliated like those of annelids; cilia are altogether lacking in all arthropods. 

 The body wall of Peripaius is covered by a thin, flexible cuticle rather than 

 a hardened exoskeleton. Although a tracheal system is present as in many 

 arthropods, in onychophores the tracheae arise in tufts from the inner ends 

 of spiracular openings which are scattered over the surface of the body, not 

 restricted to segmental intervals. 



Taken as a group, onychophores may be considered as persistent, little- 

 changed representatives of an ancient stock of animals which evolved from the 

 annelid-arthropod stem independently of the groups that later gave rise to 



Fig. 15.43. Phylum Onychophora: general features of Peripaius capensis, natural size. Note 

 the srrialK- repeated, lobe-like, clawed appendages of the trunk, which are characteristic of 

 animals in this phylum. (Redrawn from A. Sedgwick, 1888, Studies from the Morphological Labora- 

 tory in the University of Cambridge, vol. 4.) 



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