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 shghtly, it seems 
unnecessary to compheate matters with sub-familes 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 jot 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. 
