316 MR. W. B. S. BENHAM ON THE MUSCULAR 



a great deal, and becomes triangular in cross section, with its base horizontal ; this is 

 pierced by a lateral canal, in which runs an artery supplying the movable spines 

 (PI. LXXVI, figs. 10, 11). This canal is open behind into the metasoma, and in front 

 curves along the oblique anterior edge of the abdominal carapace and opens into the 

 prosomatic cavity. 



On each side of the median arch mentioned above, are situated six entapophyses 

 (PI. LXXVI. Ent- to Enf), smaller than the pair in the prosomatic carapace, but with 

 the same direction ; each is smaller than its predecessor, the last being very short. 

 Five of these belong to the mesosoma, the first of the six fused segments of which has 

 no entapophysis : the last belongs to the metasoma. They all vary a good deal in shape, 

 though in general they are flattened laterally. Several muscles are attached to each of 

 these, as will be seen later on. 



Along the anterior edge is a similar chitinoiis network to that found on the proso- 

 matic carapace. This is continuous along the line of the entapophyses, leaving spaces 

 for attachment for muscles, and is found elsewhere. 



II. The Sternites (seen from within). (Plate LXXII.) 



1. Prosomatic Region. — Outside the attachments of the limbs, whose basal joints form 

 the sides of the prosomatic region, the ventral hard chitinous portion of this region 

 curves outwards and downwards to join the dorsal portion (lateral convexity) : thus the 

 floor of the prosoma, at the sides, is convex from within, and there is only a very shallow 

 space between tergite and sternite. 



Anteriorly, in the median portion, there is a triangular flat portion, the subfrontal 

 area {Sfa), which forms the floor of a much deeper space between tergite and sternite, 

 in which is lodged the muscular stomach. The sides of this triangular space curve 

 upwards and outwards, forming a continuation with the general convex sternal portion 

 outside the coxal attachments. The apex, which is median and posterior, has an almost 

 vertical wall, which rises for a short distance and is then continued as a chitinous mem- 

 brane backwards. 



The median portion of the floor of the prosoma, the real sternal region of this division 

 of the body, above which are lodged the various organs, is principally membranous, with 

 certain chitinous sclerites here and there. 



The mouth is situated in almost the centre of this part, between the bases of the 

 third pair of prosomatic limbs. The oesophagus («) is of chitinous membrane, and has 

 harder ridges along it, which radiate along the floor of the prosoma towards the coxee 

 of the limbs. 



In front of the mouth, and between the coxae of the first pair of prosomatic 

 appendages, is an ovate piece of hard chitin, the sclerite of the " camerostoma " of 

 Latreille, which forms a sort of upper lip (Cam). 



