XI 



PHYLUM ARTHROPODA 



617 



In the Xiphosura or King-crabs (Fig. 510), the body consists of 

 two well-marked regions cephalotliorax and abdomen. The former 

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

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

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

 together, being covered dorsally by a continuous abdominal cara- 

 pace. At the posterior end is attached a very long, narrow, 

 caudal spine. The anterior 

 appendages (Fig. 511) re- 

 semble those of the Scor- 

 pion. In front of the 

 mouth is a pair of short, 

 three-jointed, chelate ap- 

 pendages, the chelicerce (T), 

 at the sides of a labrum 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 abdomen are flat plates, 

 which are united together 

 in the middle line and 

 together form the broad 

 operculum (operc.), over- 

 lapping all the posterior 

 appendages ; on its pos- 

 terior 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. A labrum (rostrum) lies in front 

 of the mouth, and between the sixth pair of appendages is a 

 pair of processes, the chilaria. 



In the Eurypterida (Fig. 512) 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 

 narrow pointed telson. There are usually five pairs of legs sur- 



FIG. 510. Limulus. Dorsal aspect. 

 (After Leuckart.) 





