332 ARACHNOIDEA AND PAL&OSTRACA. 



The recently discovered antennae of Trilobites, together 

 with the markedly biramose character of some of their 

 limbs, suggest an affinity with Crustacea, but, on the other 

 hand, the affinities of the Xiphosura seem to be distinctly 

 Arachnoid. 



Order i. Xiphosura. 



There is one living genus, the King-crab or Horseshoe- 

 crab (Limulus). 



The King-crab lives at slight depths off the muddy or 

 sandy shores of the sheltered bays and estuaries of North 

 America, from Maine to Florida, in the West Indies, and 

 also on the Molucca Islands, etc., in the far East. The 

 body' consists of a vaulted cephalothorax shaped like a 

 horseshoe, and an almost hexagonal abdomen ending in a 

 long spine. Burrowing in the sand, Limulus arches its 

 body at the joint between cephalothorax and abdomen, and 

 pushes forward with legs and spine. It may also walk 

 about under water, and even rise a little from the bottom. 

 It is a hardy animal, able to survive exposure on the shore, 

 or even some freshening of the water. Its food consists 

 chiefly of worms. 



The King-ciab is interesting in its structure and habits, and also 

 because it is the only living representative of an old race. .Since Prof. 

 Lankester published in 1881 a famous paper entitled " Limulus an 

 Arachnoid," it has been generally, though not unanimously, recognised 

 that the King-crab's relationships among modem animals are with 

 Arachnoidea, not with Crustacea. 



The hard, horseshoe - shaped, chitinous cephalothoracic shield is 

 vaulted, but the internal cavity is much smaller than one would at first 

 sight suppose ; the well-defined abdomen shows some hint of being 

 divisible into meso- and metasoma ; the long, sharp spine is (like the 

 scorpion's sting) a post-anal telson. 



On the concave under-surface of the cephalothorax there are six (or 

 seven) pairs of limbs, as in spiders and scorpions — 



(1) A little pair of three-jointed chelicerce in front of and bent 

 towards the mouth. (They are chelate in the female, 

 '•-"* ' simply clawed in the male. ) 



(2-6) Five pairs of six-jointed walking legs, the bases of which 

 surround the mouth, and help in mastication. The last of 

 these ends in two flat plates, which help in digging. The 

 others are usually chelate, except the first in the male. 

 (7) Then follows on the abdomen a double "operculum" over- 

 lapping the rest. The genital apertures lie on its posterioi 

 surface. Some refer this operculum to the cephalothorax. 



