I'^CJLOPODA. 261 



the Apus properly so called. The same natmalist has figured 

 another species, Apus Montagui, Edinb. Encyclop. Suppl. 

 I. XX. 



ORDER II. 



PiECILOPODA. 



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. 



The 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 pre- 

 sents two eyes when those organs are distinct. Two of their antennae 



Cheliceres, Lat. — form a forceps in several, and fulfil its functions. 



Most of them have twelve feet *, and nearly all the remainder have 

 either ten or twenty-two. Their usual habitat is on aquatic 

 animals, and most commonly on fishes. 



We divide this order into two families f. 



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 cover- 

 ed with small spines and perform the office of jaws ; there are twenty- 



* Fourteen in several, according to Leach ; those which he considers as the two 

 first, ho-wever, appear to me to be two inferior antennae. The Arguli, which 

 seem to be the most favoured subgenus with respect to locomotiou, have but twelve 

 feet. 



t In my Fam. Nat. du Rcgne Anim. they form two orders. 



