294 ARTHROPODS. 



from Maine to Florida and the West Indies, and also on the 

 Molucca Islands, etc., 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 of worms and small animals found 

 in the sand. There are two or three different species (Z. 

 polyphemus from N. America, L. moluccanus from the East). 

 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 horse-shoe-shaped hard, chitinous cephalothoracic shield is 

 vaulted, but the internal cavity is much smaller than one vi^ould at first 

 sight suppose ; the abdomen is all of one piece, also covered by a shield ; 

 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 cheliceroe in front of and bent towards the 

 mouth. (They are chelate in the female, simply clawed in 

 the male. ) 



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

 mouth and help in mastication. (The last of these ends in 

 two flat spines, which, along with others a little higher up, 

 help the animal in walking on soft sand. The other 

 appendages are usually chelate.) 



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

 lapping the rest. (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 are said by some to show hints of the exopodite 

 and endopodite structure characteristic of Crustaceans.) 



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

 sternite serving for the attachment of muscles. 



