CHAPTER XI 



THE SUBPHYLA ONYCHOPHORA AND 

 TRILOBITA 



The two groups of animals with which this chapter deals both present 

 in an apparently primitive condition features which are characteristic 

 of the phylum Arthropoda. One at least of them existed in the Palaeo- 

 zoic period. For these reasons, each of them has been regarded as 

 giving indications concerning the ancestry of the Arthropoda. Where- 

 as, however, the Trilobita are clearly related to the Crustacea and less 

 closely to the other subphyla, the Onychophora are, as has been stated 

 above, widely divergent from the rest of the Arthropoda. Some 

 authorities, indeed, prefer to treat this group as an independent 

 phylum. It must at least be regarded as representing a branch which 

 parted at a very early date from the main arthropod stock. The trilo- 

 bites are indisputable arthropods, on the line of descent which gave 

 rise to the Crustacea and perhaps to other subphyla. 



SUBPHYLUM ONYCHOPHORA 



Tracheate Arthropoda with a thin, soft cuticle and a body wall con- 

 sisting of layers of circular and longitudinal muscles ; head not marked 

 off from the body, consisting of three segments, one preoral, bearing 

 preantennae, and two postoral, bearing jaws and oral papillae respec- 

 tively, also with eyes which are simple vesicles ; the remaining segments 

 all alike, the number varying according to the species, each bearing a 

 pair of parapodia-like limbs which end in claws and contain a pair of 

 excretory tubules ; stigmata of the tracheal system scattered irregularly 

 over the body ; cilia present in gut and genital organs ; development 

 direct. 



The animals which constitute this very important class are few in 

 number and uniform in structure, all being placed in the genus Peri- 

 patus divided into many subgenera (Fig. 203). They are distri- 

 buted discontinuously over the warmer parts of the world and occur 

 in very retired positions as, for instance, beneath the bark of dead 

 trees and under stones. They have a superficial resemblance to other 

 crawling animals which are found in the same places, like myriapods, 

 slugs and earthworms, and until their anatomy was well known were 

 classed, by different investigators, with all three of these. Certain of 

 the characters of Peripatus such as the feebly developed sense organs, 

 the simple structure of the jaws and feet and the soft skin may be 



