32 1 6 THE JOINTED ANIMALS 



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 

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

 instance, covering a span of nearly two feet from toe to 

 SHORE SPIDER (enlarged), toe. None are able to swim, but all crawl slowly among 



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 be- 

 ing 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 intermedi- 

 ate 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 specialized 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 horseshoe crabs 

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

 xistmg orms j^y j s arme( j 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 sup- 

 ports 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 cephalo- 

 thorax, which is covered above with an enormous carapace, having its border semi- 

 circular 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 com- 

 pound 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 last nipper-like, the last or 



