PJBCiLUl'ODA. 261 



the Apus properly so called. The same naturalist has figured 

 another species, Apus Montagu*, Edinb. Encyclop. Suppl. 



I , A .\ 



ORDER II. 



P^CILOPODA. 



The Ptecilopoda are distinguished from the Branchiopoda by the 

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

 unit-terminate number are ambulatory, or fitted for prehension; while 

 the others, lainrlliform or pinnate, are branchial and natatory. It is 

 ]>rinril>ally, 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 inartieulated 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- 

 nts 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 

 i-ither 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 ih<* second by several characters: 

 tin-re is no siphon . tin- 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 



appear to me to be two inferior antennae. The Arguli, which 



i to he the roost favoured subgenus with respect to locomotion, have but twelve 



f In my Fam. Nat. du R N gne Anim. thry form two orders. 



