ARTHROPOD A 



173 



Supplementary Order Xiphosura 



The Xiphosura (Gr. £i(f>o<;, sword, and ovpd, tail), called com- 

 monly the king crabs or horseshoe crabs, constitute an order in- 

 termediate in a way between the Crustacea and the succeeding 

 subtype, the Arachnida, so that it seems best to take it up at this 

 point. There is only a single genus, Limulus (Fig. t66), with 

 very few species, and it occurs in salt water on sandy shores along 

 the eastern coast of the United States, on the West Indies, and 

 on some of the Mast Indies. 

 They grow to a considerable 

 size, sometimes becoming 

 some thirty centimeters 

 broad and at least twice as 

 long. The body consists of 

 three parts : the horseshoe- 

 shaped cephalothorax, eon- 

 vex on the dorsal side 

 and concave beneath, the 

 abdomen of fused segments, 

 and the stiff, swordlike tail, 

 which is probably to be 

 regarded as a portion of 

 the last segment of the 

 abdomen. 



The cephalothorax bears 

 the mouth on its ventral 

 surface surrounded by six 

 pairs of appendages ; the 

 first pair is small, the others 

 are larger and serve as walking feet ; antenna- are apparently 

 absent entirely. At the posterior end of the cephalothorax is 

 a seventh pair of appendages, regarded by some zoologists as 

 belonging to the thorax and by others as constituting the first 

 pair of abdominal appendages. This pair is broad and platelike, 

 the two meeting on the median line of the body and forming a 

 sort of cover — hence called the operculum- to the succeeding 

 five pairs of abdominal appendages, which are also platelike. 



FIG. 166. Limulus polyphemus, the horseshoe 

 crab, adult female , ai tuai li ngth, forty-six centi- 

 meters. (Photographed from .1 third specimen 

 by the author.) 



