480 



CRUSTACEA. 



there are five pairs of long spiny legs, of which the last is much the 

 largest and ends in a broad swimming-fin. Some of the anterior 

 appendages may be armed with a chela, The resemblance of the true 

 EurypteridcB (in the general shape of their body) to the Scorpionidce 

 is very striking, while the genus Hemiaspis presents affinities to the 

 Pcecilopoda. The most important forms are: Eurypterus pygmceus 

 Saft., Devonian strata, Pterygotus anglicus Ag., four feet long, from 

 the upper Silurian (fig. 373). 



FIG. 373. Eurypterus remtpes after Nieszkowski. a, Dorsal view ; b, ventral view; O, eyes; 

 St. caudal spine ; -ff, hypostome. 



Order 2. XIPHOSURA.* 



Gigantostraca whose body is divided into three parts, which are 

 movably articulated together ; a large shield-shaped cephalo-thorax, an 

 abdomen ivithfive pairs of lamellar feet and a long movable caudal spine. 



The large body of these Crustacea is covered with a strong chiti- 



* C. Gegeiibaur, " Anatomische Untersuchung cincs Limulus, mit besonderer 

 Beriicksichtigung der Gewebe," Alliandl. der naturforsch. Gesellschaft zu 

 Halle, IV., 1858. Packard. ' ; The Development of Limulus Polyphemus," Svc. 

 of Nat. Hist., 1870. A. M. Edwards, " Rechcrches sur 1' anatomic des Limules," 

 .4n7?,. xc. nat. V c Ser. Tom. XVII.. 1872-1873. [E. E. Lankester, "Limulus 

 an Arachnid." Quart. Journ. Mic. Soc., vol. xxi.J 



