



318 CRUSTACEA. 



MONOCULUS, Lin. 

 Which is now divided into two great sections and various subgenera. 



ORDER II. 

 PJECILOPODA(l). 



The Paecilopoda are distinguished from the Branchiopoda by the 

 diversity in the form of their feet, among the anterior of which an 

 indeterminate number are ambulatory, or fitted for prehension; 

 while the others, lamelliform or pinnate, are branchial and natatory. 

 It is principally, however, by the absence of the usual mandibles and 

 jaws that they are removed from all other Crustacea. Sometimes 

 these parts are replaced by the spinous haunches of the first six pairs 

 of feet; and sometimes the organs of manducation consist either of 

 an external siphon in the form of an inarticulated rostrum, or of 

 some other apparatus fitted for suction, but concealed or slightly 

 apparent. 



Their body is almost always, either wholly, or for the greater por- 

 tion, invested with a shell in the form of a shield, consisting of a 

 single plate in most of them, and of two in others, which always 

 presents two eyes when those organs are distinct. Two of their 

 antennas Chelicerse, Lat. form a forceps in several, and fulfil its 

 functions. Most of them have twelve feet, and nearly all the re- 

 mainder have either ten or twev>ty-two. Their usual habitat is on 

 aquatic animals, and most commonly on Fishes. 



We divide this order into two families. 



FAMILY I. 



XYPHOSURA. 



This family is distinguished from the second by several characters: 

 there is no siphon; the haunches of the first six pair of feet are 

 covered with small spines and perform the office of jaws; there are 



(1) Various-footed. 





