XIPHOSURA 401 



with a number of movable projections called the pushers, because they 

 are used by the animal in pushing- itself through the sand and mud. The 

 first pair lie in front of the mouth, and are called the mandibles or 

 chelicerae: they are much smaller than the others, but have the same 

 sliape. The basal joints of the remaining five pairs of legs are spiny 

 and assist in chewing the food. The abdomen bears six pairs of appen- 

 dages, none of which are leg-like. The first pair are called the operculum 

 and form together a broad plate which falls over and covers the remain- 

 ing five pairs; these are also plate-like and bear on their hinder surfaces 

 numerous thin gills. The males are smaller than the females and differ 

 from them in the structure of the second pair of appendages, the claw of 

 which is a thickened structure, of use in holding on to the shell of the 

 female while pairing. The j)aired genital openings are at the base of the 

 operculum. The telson may be as long as the rest of the body. 



The dorsal surface of the cephalothorax bears two large lateral com- 

 posite eyes and a pair of small median ones. The dorsal surface of the 

 abdomen is flattened and bears a row of movable, spines on each lateral 

 edge. The internal anatomy is essentially arachnid in character.* 



The king crab lives in shallow water along the shore, where it bur- 

 rows in the sand and mud and eats worms and other small animals. It 

 comes to sandy beaches in the early summer to breed, and lays its eggs 

 in depressions it makes in the sand. The embr3'o as it emerges from the 

 egg has a resemblance to a trilobite and lacks the spine-like telson and 

 the abdominal appendages. The animals, although of large size, have 

 little economic importance. They are, however, sometimes fed to chick- 

 ens and pigs. 



History.— The American Limulus was first made known in 1590 by 

 Thomas Harriot m his description of the animals and plants of Virginia. 

 The Asiatic species became known during the 17th century, the dried shells 

 having frequently been brought to Europe as curiosities. Until quite 

 recently zoologists have placed Limulus among the crustaceans. Latreille, 

 however, in 1808 called attention to its peculiar structure and created for 

 it the separate order Xiphosura, and Straus-Diirckheim in 1829 empha- 

 sized its resemblance to arachnids. This idea, however, gained ground 

 very slowly, although Huxley and von Beneden both spoke in favor of 

 it, and it was not until after Lankester's demonstration in 1881 that 

 Limulus was finally accepted as an arachnid. f Another question of rela- 

 tionship with which Limulus has to do is whether the primitive arachnid 



* See "The Embryology of Limulus," by J. S. Kingsley, Jour. Morph., Vol. 7, p. 

 35, and Vol. 8, p. 19.5, 1892-3. "Studies on Limulus," by W. Patten and W. A. Reden- 

 baugh, Jour. Morph., Vol. 16, p. 1 and p. 91, 1900. 



t See "Limulus an Arachnid," by E. R, Lankester, Quart. Jour. Mic. Sci., Vol. 

 21, 1881. 



