P^ECILOI'ODA. 
261 
the Apus properly so called. The same naturalist has figured 
another species, Apus Montagui, Edinb. Encyclop. Suppl. 
ORDER 11. 
P^CILOPODA. 
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. T wo 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, however, appear to me to be two inferior antenuse. The Arguli, which 
seem to be the most favoured subgemis with respect to locomotion, have but twelve 
feet. 
f In mv Fam. Nat. du Regue Anim. they form two orders. 
