PHYLUM ARTHROPODA 



559 



it of Apus, 011 the last cephalic segment. Ventrally the carapace is con- 

 med, as in Apus, into a sub-frontal plate (B, s.f.p], to the posterior edge of 

 which is attached a large labrum (Ibr). 

 The posterior angles of the carapace are 

 often produced into spines. 



The thorax (th) is composed of a 

 variable number (2-29) of movably 

 articulated segments, which are com- 

 monly trilobed, consisting of a median 

 region or axis, and of lateral pleura (pi) 

 often produced backwards and down- 

 wards into spines. The abdomen is 

 covered by a caudal shield or pygidium 

 (ji), formed of a variable number of 

 fused segments. Owing to the mobility 

 of the thorax, the Trilobites were able 

 to roll themselves up like Wood-lice (B). 



The appendages are very imperfectly 

 known. Quite recently a single pair of 

 antenna? (Fig. 442) has been shown to 

 exist in one species, probably attached 

 to the sub-frontal plate. Four pairs 

 of leg-like cephalic appendages have 

 been demonstrated, and the thorax 

 bears slender biramous legs consisting of 

 endo- and expedite, and bearing spiral 

 gills. Similar limbs are present on the 

 abdomen. 



The larvae of several species of 

 Trilobites have been found in the fossil 

 state. In some of these the body con- 

 sists only of carapace and pygidium in 

 the youngest stages, 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 iiauplii. 



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

 ist affinities seem to be, on the whole, with the Phyllopoda. 



-Fio. 441, bis. Triarthrus beckii, dorsal 

 aspect, showing antennae and thoracic 

 abdominal 



and abdominal appendages. 

 Bernard, after Beecher.) 



(From 



j 



CLASS II. ONYCHOPHO 





The class Onychopbora comprises only the aberrant arthropod 

 genus Peripatus, 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. 442) is a cater- 

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

 into segments : it has a fairly well-marked head, and a series (14-42 





