384 ARTHROPOD A. 



aspis, with only nine segments and the telson behind the 

 carapace, is exclusively Upper Silurian. Lastly, Pseudoniscus, 

 with the same number of free segments, is found in the pas- 

 sage-beds between the Upper Silurian and Devonian. In 

 conclusion, it is interesting to note that these ancient Crus- 

 taceans present many larval features, resembling the larvae 

 of the Decapoda, especially in the fact that the hinder por- 

 tion of the body is composed of free segments, which carry 

 no appendages. 



SUB-ORDER II. XIPHOSURA (PoscUopoda). 



" Crustacea having the anterior segments welded together to 

 form a broad convex buckler, upon the, dorsal surface of ivhich 

 are placed the compound eyes and ocelli; the former sub- 

 centrally', the latter in the centre in front. The mouth is 

 furnished with a small laforum, a rudimentary metastoma, and 

 six pairs of appendages. Posterior segments of the body more 

 or less free, and bearing upon their ventral surfaces a series of 

 broad lamellar appendages; the telson, or terminal segment, 

 ensiform." (Henry "Woodward.) 



The only living members of the Xiphosura are the Limuli, 

 commonly known as King-crabs or Horse-shoe Crabs. The 

 anterior portion of the body is covered by a broad horse- 

 shoe-shaped buckler (fig. 238), the upper surface of which 

 bears a pair of larval and a pair of compound eyes. On the 

 lower surface of the carapace is placed the aperture of the 

 mouth, surrounded by six pairs of limbs, the bases of which 

 are spinous, and officiate as jaws, whilst their terminations 

 are converted into chelae or nipping-claws. The first pair of 

 appendages is placed in front of the mouth, and represents 

 the antennae, so that the antennae of the King-crabs are 

 chelate. Behind the cephalic buckler comes a second shield, 

 composed by six amalgamated segments, below which are 

 carried the reproductive organs and branchiae, the former 

 protected by a thoracic plate or " operculum," the latter 

 borne by five pairs of lamellar appendages. Lastly, articu- 

 lated to the posterior margin of the abdominal shield, is a 

 long sword-like spine or "telson" (fig. 238, t). 



