xi PHYLUM ARTHROPOD A 591 



The larvse of several species of Trilobites have been found in the fossil 

 state. In some of these stages the body consists only of carapace and 

 pygidium in the youngest, and the thoracic segments are subsequently 

 intercalated in regular order. In other species the earliest stage has the 

 form of a rounded plate, the posterior portion of which elongates and segments 

 to form the thorax and abdomen. Nothing is known of the larval appendages, 

 and none of the stages hitherto discovered can be considered as nauplii. 



The precise systematic position of the Trilobites is uncertain, but their 

 nearest affinities seem to be, on the whole, with such Branchiopoda as Apus ; 

 the relationship is, however, by no means a close one. 



CLASS II.-ONYCHOPHORA. 



The class Onychophora comprises only the aberrant arthropod 

 Peripatus, with several sub-genera (or closely allied genera), which 

 differs very widely in certain important features of its organisation 

 from all the rest of the Arthropoda, and in some respects enables 

 us to bridge over the interval between the latter and some of the 

 lower phyla, more particularly the Annulata. 



General external features. Peripatus (Fig. 486) is a cater- 

 pillar-like animal of approximately cylindrical form, and not divided 



FIG. 486. Peripatus (Peripatopsis) capensis, lateral view. (From Balfour. 



into segments ; it has a fairly well-marked head, and a series (14 

 42, according to the species) of pairs of short stumpy appendages. 

 The integument is thrown into a number of fine transverse wrinkles, 

 and is beset with numerous conical papillae, each capped with a 

 little chitinous spine. The head (Fig. 487) bears a pair of antennae, 

 a pair of eyes, a pair of jaws, and a pair of short processes known 

 as the oral papillce. The antennas are made up of a number of 

 short rings bearing minute spines. The eyes are constructed 

 somewhat after the model of the chaetopod eye as described on 

 p. 437. On the surface of the oral papillae are situated the apertures 

 of a pair of glands the slime-glands. Each jaw is composed of 

 two curved, falciform, pointed, chitinous plates, the inner toothed 

 on its posterior concave edge ; they lie at the sides of the mouth 

 enclosed by a circular lip. The jaws, as well as the oral papillae, 

 are developed as modified limbs. 



The legs are not jointed, but rows of papillae give them a ringed 

 appearance ; each consists of a conical proximal part and a small 

 distal part oifoot, the latter terminating in a pair of horny claws. 



The ventral surface is reddish in colour, the dorsal darker : 

 the latter presents an elaborate pattern which varies greatly in 

 different individuals produced by minute mottlings of various 



