KING-CRABS. 249 



backwards between the last pair of legs, is the abdomen, which is reduced to a 

 mere tubercle or rod-like process. The greater part of the body-cavity is occupied 

 by the stomach, which sends prolongations almost down to 

 the extremities of the four pairs of walking -legs. No 

 breathing-organs are known. 



The sea- spiders are exclusively marine, and range 

 from shallow water to depths of sixteen hundred fathoms 

 or more. The conditions of life in the deep sea have by 

 no means a dwarfing effect upon them, since the species 

 living in the abysses of the ocean attain a size never 

 equalled by those frequenting the coast. Some of the shore-spider (enlarged). 

 former are of a very large size ; Colossendeis gigas, for 



instance, covering a span of nearly two feet from toe to toe. None are able to swim, 

 but all crawl slowly amongst the branches of seaweed. The embryo emerges from 

 the egg as a larva, provided with a beak and three pairs of appendages, representing 

 the short anterior three pairs of the adult ; the four pairs of great locomotor limbs 

 being subsequently produced by outgrowths from a posterior elongation of the body. 



The King-Crabs, — Class Gigantostraca. 



In many respects the representatives of this class occupy a position intermediate 



between the Scorpions and Spiders and the Crustaceans. From the fact that they 



are marine and breathe by means of gills, they were formerly always classified with 



the Crustaceans ; but a large amount of evidence has been brought forward to show 



that whereas the earliest kinds are related to the primitive Crustaceans, the more 



specialised kinds are strikingly like some of the Scorpions. The class contains 



three orders, named Xiphosura, Merostomata, and Trilobita. The last two of these 



are now entirely extinct, and the first named nearly so, since it is represented 



at the present day by only a single genus, the king-crabs or horse-shoe crabs 



(Limulus). In the existing: group, forming the order Xiphosura, the 

 Existin 0, Forms o o j. > o j. 



' body is armed behind with a long spike-like tail, movably articulated 



to the middle of the hinder border of the abdomen. The abdomen consists of a 

 large unsegmented pentagonal plate, armed on each side with six movable spines, 

 and hollowed out below to receive six pairs of large flattened limbs, attached to 

 the anterior part of its lower surface. "With the exception of the first, each 

 limb supports on its hinder surface a bunch of fine branchial plates, arranged one 

 above another like the leaves of a book. In front of the abdomen comes the 

 cephalothorax, which is covered above with an enormous carapace, having its border 

 semicircular and its hinder angles acutely produced. The carapace is furnished 

 above with four eyes, two being small and simple ocelli, situated close together 

 some little distance behind the front border, while the others are large kidney- 

 shaped compound eyes, placed at a corresponding distance from the lateral 

 margin. The great size of the carapace is due to the prolongation of its edges 

 into a wide sloping shelf-like expansion, concealing the walking limbs. Of the six 

 pairs of the latter, the first are placed in front of the mouth, and are short, three- 

 jointed nippers ; while the rest are longer, generally six-jointed, and all but the 



