338 ARACHNOIDEA AND PAL^OSTRACA. 



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, &c., in the far East. The 

 body consists of a vaulted cephalothorax shaped like a 

 horse-shoe, 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 Crab is interesting in its structure and habits, and also 

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

 Lankester published in 1881 a famous paper entitled "Limulus an 

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

 that the King Crab's relationships among modern animals are with 

 Arachnoidea, not with Crustacea. 



The hard, horse-shoe-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 : 



(i) A little pair of 3-jointed chelicerse in front of and bent 

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

 simply clawed in the male.) 



(2-6) Five pairs of 6-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 

 other appendages 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 posterior 

 surface. Some refer this operculum to the cephalothorax. 



(8-12) Under the operculum lie five pairs of flat plates bearing 

 remarkable respiratory organs ("gill books"). These 

 appendages show hints of the exopodite and endopodite 

 structure characteristic of Crustaceans. At any rate in 

 the young they serve also as swimming organs. 



As in the scorpion, there is an internal skeletal structure, or endo- 

 sternite, lying between the gullet and the nerve ring, serving for the 



