340 ARACHNIDA. XIPHOSURA 



The prosoma or cephalothorax is covered dorsally by a 

 large crescentic or nearly semicircular carapace (fig. 145, l), 

 which is very convex above and carries on its upper sur- 

 face two pairs of eyes, one compound and lateral (5), the 

 other simple and median (4). The large compound eyes 

 are near the middle of the lateral parts of the cephalo- 

 thorax ; the small simple eyes are close together in the 

 middle line, near the anterior margin. The abdomen is 

 more or less hexagonal in outline and is movably articu- 

 lated with the cephalothorax ; both have two longitudinal 

 furrows on the dorsal surface, dividing a narrow axial 

 part from a broad lateral portion on each side, thus giving 

 a superficial resemblance to a Trilobite. The mesosoma 

 forms the main part of the abdomen and is composed of 

 six fused segments, the segmentation being shown by 

 grooves on the dorsal surface, and by the six movable 

 spines borne on each side. The small posterior part of 

 the abdomen without grooves represents the metasoma. 



The prosoma (cephalothorax) carries six pairs of ap- 

 pendages concealed in the concavity of its under surface ; 

 the anterior pair (fig. 144, 1) (chelicerce) only are in front 

 of the mouth and are small, three-jointed appendages with 

 chelae. The other five pairs (2 — 6) are the long, six- 

 jointed walking-legs and are placed at the sides of and 

 just behind the mouth ; most of them (except the last 

 pair) end in chelae, and their basal joints (except in the 

 sixth pair) are spinose and function in mastication. Be- 

 hind the mouth are a pair of small unjoin ted processes, 

 the chilaria, which represent a seventh pair of append- 

 ages. The abdomen carries six pairs of plate-like 

 appendages ; the anterior pair are united, forming what 

 is known as the genital operculum (7), on the posterior 

 surface of which are the genital openings. The operculum 



