260 



ZOOLOGY 



Ancient 



Xiphosura 



Subclass Xiphosura 



Aquatic animals, 

 known as horseshoe 

 crabs, on account of the 

 outline of the cephalo- 

 thorax. They differ from 

 the true arachnids in pos- 

 sessing gills. The name 

 Xiphosura (sword tail) 

 refers to the swordlike 

 (or spinelike) tail. The 

 horseshoe crabs, or king 

 crabs, which of course 

 are not crabs at all, grow 

 to a large size, and may 

 be found in abundance 

 in the sea along our 

 Atlantic coast. Large 

 specimens are about a 

 foot and a half long, and 

 the color is dark brown. 

 Although the individuals 

 are many, all belong to a single species, Limulus (or 

 Xiphosura) polyphemus. Several others occur in Asiatic 

 seas, but the group is a very small one in the existing 

 fauna, evidently an ancient type represented by a few 

 survivors. At Mazon Creek, Illinois, nodules of the 

 carboniferous period are found containing fossil animals 

 and plants, and among them a primitive king crab 

 called Euproops dance. This animal, which lived more 

 than ten million years ago, inhabited fresh water; so 

 it is not unlikely that the Xiphosura had their origin in 

 inland waters. 



Photograph by W. P. Hay 



FIG. 73. A horseshoe crab (Limulus) show- 

 ing the ventral surface; about natural 

 diameter. 



