376 



PHYLUM ARTHROPODA. 



op. 



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-crab is interesting in 

 its structure and habits and also 

 because it is the only living repre- 

 sentative of an old race. 



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 meta-soma; 

 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 chelicerae in front 

 of and bent towards the 

 mouth. 



(2) A pair of pedipalps lateral 



to the mouth. 



(3-6) Four pairs of walking 

 legs, the bases of which 

 surround the mouth, and 

 help in mastication. Be- 

 hind these, still on the cephalothorax, there is a pair of small 

 appendages called chilaria. 

 Then follows on the abdomen a double "operculum" with the 



genital apertures on its posterior surface. 



Under the operculum lie five pairs of flat plates bearing remark- 

 able respiratory organs ("gill-books"). These appendages 

 show hints of the exopodite and endopodite structure character- 

 istic of Crustaceans. 

 Each " gill-book " looks like a much-plaited gill, or like a book with 



FIG. 199. Limulus or King-crab. 



ek, t Chelicerae ; op., operculum ; 

 a., anus. 



