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 related rather closely to the Crustacea 

 and more distantly 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 repre- 

 senting a branch which parted at a very early date from the main 

 arthropod stock. The trilobites 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 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. 214). They are distributed 

 discontinuously over the warmer parts of the world and occur in very 

 retired positions which are permanently damp as, for instance, be- 

 neath 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 v/ere classed, by different investigators, with all three 

 of these. Certain of the characters of Peripatus such as the feebly de- 

 veloped sense organs, the simple structure of the jaws and feet and 

 the soft skin may be linked with the environment in which they lurk 



