V XIPHOSURA 417 



hy a shield, a thorax consisting of 5 or 6 free (rarely fused) rings, and 

 an abdomen consisting of 3 or more segments, and followed by a 

 strong caudal stylet. The cephalic shield often has 2 compound 

 lateral eyes, ocelli are wanting. Two dorsal longitudinal furrows give 

 the thorax an appearance like that of the Trilobites. The extremities 

 are unknown. 



Bunodes, Hemiaspis, Belinurus. 



IV. The Xiphosupa (Poeeilopoda, Limulidse). 



The body, which is covered by a hard and thick chitinous carapace 

 falls into 2 principal divisions, the cephalic shield (cephalothorax) and 

 the hind body (abdomen). The hind body is followed by a large and 

 long post-anal caudal spine which can move independently of the 

 body. The cephalothorax is very large, almost crescent-shaped, with 

 2 lateral horns directed backwards. On its dorsal side are the 2 

 compound lateral eyes, and in front of them, nearer the median- line, 2 

 ocelli. The flat segmented abdomen, which articulates -ndth the 

 cephalothorax, is almost hexagonal. 



The upper side of the cephalothorax is arched, and the under side 

 is coiicave. 



L\mbs (Fig. 284:). — There are 7 pairs of limbs on the cephalothorax. 

 The nl\ost anterior lie in front of the mouth ; they are small and end in 

 pincers. They are innervated from the anterior part of the oesophageal 

 commissure, and (if a comparison with Crustacea is allowed) should 

 perhaps correspond with the 2d pair of antennse, while limbs 

 corresponding with the anterior antennae are wanting. The 1st pair of 

 limbs, which are called ehelieerse, are followed by 5 strong long pairs 

 of limbs, also ending in pincers. They arise at the sides of the slit- 

 like oral aperture, and each has a masticatory ridge on the basal 

 joint. The masticatory ridges of the 4 anterior pairs of feet are 

 armed with spines, those of the 5th pair, however, have a sharp cut- 

 ting inner edge. The 5th pair of feet is further distinguished by 

 a different formation of the terminal joint and by an appendage 

 on the basal joint, which has been considered to be an exopodite. 

 Behind the mouth there are two stylet - shaped processes, the 

 ehilaria. 



The 7th pair of limbs of the Cephalothorax, which arises at its 

 posterior edge, is quite differently formed from the preceding, and 

 agrees far more vpith the abdominal limbs. It is called the operculum, 

 and consists of 2 plates, which are united in the middle line, and 

 cover the subsequent abdominal limbs. The latter, of which there 

 are 5 pairs, are leaf-like, and shaped like the operculum. In all the 

 leaf-like feet 2 rows of areas are marked by sutures, an outer row of 

 large areas (exopodites) and an inner row of smaller areas (endopodites). 

 The last areas or joints project freely. The leaf -shaped abdominal 

 feet on each side carry on the upper surface {i.e on the surface turned 



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