666 



ZOOLOGY 



SECT. 



or for swimming ; they end usually in two olaws, between which 

 there may be discs or stalked suckers. 



In the Xiphosura or King-crabs (Fig. 548), the body consists of 

 two well-marked regions — cephalothorax and abdomen. The former 

 is covered over by a wide, dorsally convex, sub-crescentic shield or 

 carapace, bearing two large compound eyes, and two smaller simple 

 eyes. The segments of the abdomen (seven in number) are united 



together, being covered dor- 

 sally by a continuous ab- 

 dominal carapace. At the 

 posterior end is attached a 

 very long, narrow, caudal 

 spine or telson. The anterior 

 appendages (Fig. 549) re- 

 semble those of the Scor- 

 pion. In front of the mouth 

 is a pair of short, three- 

 jointed, chelate appendages, 

 the chelicercv (i), at the 

 sides of a labrum {rostrum) 

 or upper lip. Behind these 

 follow a series of five pairs 

 of legs, the bases of all of 

 which, with the exception 

 of the last, are covered 

 with spines, and have the 

 action of jaws, while the 

 extremities are for the most 

 part chelate. The first pair 

 of appendages of the ab- 

 domen are flat plates, which 

 are united together in the 

 middle line and together 

 form the broad operculum 

 (operc), overlapping all the 

 posterior appendages ; on its 

 posterior face are the two 

 genital apertures. The pos- 

 terior appendages, of which 

 there are five pairs, are thin flat plates to which the gills are 

 attached ; each of them is divided by sutures into a small inner 

 ramus or endopodite, and a larger external ramus or exopodite. 

 Between the sixth pair of appendages is a pair of processes, the 

 chilarla. 



In the Eurypterida (Fig. 550) there is a small cephalothorax 

 bearing a pair of large eyes and a pair of ocelli, and an elongated 

 segmented region containing twelve segments, followed by a 



Fin. ,'iIn Limulus 



(After Leuckart.) 



Dorsal aspect. 



