260 ARACHNIDA XIPHOSURA CHAP. 



A few years ago Pocock re-classified the Xiphosura, and his 

 classification will be found on pp. 276, 277. It will be noticed 

 that in his classification the generic name Limulus has disappeared. 

 I have, however, retained it in this article, partly because I regard 

 the name as so well established that every one knows what it 

 denotes, and partly because in a group which contains confessedly 

 very few species, differing inter se comparatively slightly, it seems 

 unnecessary to complicate matters with sub-families and new 

 names. 



Looked at from above a Limulus presents a horse-shoe-like 

 outline, from the posterior end of which projects a long spine. 

 It is often called in America the Horsefoot-crab, but its common 

 or vulgar name is the King-crab. Across the middle of the body 

 is a joint, and this joint separates the prosoma from the meso- 

 and meta-soma which are in King-crabs fused together. The 

 prosoma comprises all the segments up to and including the 

 segment which carries the chilaria ; * the mesosoma begins with 

 the segment bearing the genital pores, and ends with the last 

 segment which bears appendages ; the metasoma comprises all the 

 segments posterior to the last segment which carries appendages. 

 The prosoma corresponds with the " cephalothorax " of some 

 authors, and the meso- plus the meta-soma are equivalent to 

 their " abdomen." 



Dorsally, then, the prosoma is a vaulted structure with a 

 smooth, horse -shoe -shaped anterior and lateral margin. Its 

 posterior edge, the line where the meso- plus the meta-soma are 

 hinged, is a re-entrant bay with three sides. The meso- and 

 meta-soma are in the King-crabs fused together and form a 

 hexagon. Three sides of this hexagonal double region form the 

 hinge, two form the lateral margins and slope inwards ; these 

 bear six fused and six jointed spines which have a segmental 

 value. The sixth or posterior side is indented, and its concavity 

 forms the area to which the large postanal, unsegmented tail or 

 spine is hinged. 



The whole body is covered by a smooth chitinous sheath 

 varying from sage-green to black in colour, and it is kept very 

 clean, probably by some excretion which hinders various sessile 

 animals attaching themselves to it as they do, for instance, on 



1 This segment, though present in embryo Scorpions, has disappeared in the 

 adults of those animals. 



