360 
CLASS CRUSTACEA. 
ber, are ambulatory, or adapted for prehension ; and the others 
lamelliform or pinnated, are branchial and natatory. But it is 
more particularly in the absence of the usual mandibles and 
jaws, that they differ from all the other Crustacea; sometimes 
these parts are replaced by the haunches, furnished with spines 
of the first six pairs of feet ; sometimes the organs of mandu- 
cation consist, either in an external siphon, in the form of 
an inarticulate beak, or in some other instruments adapted for 
suction, but concealed, or scarcely distinguishable. 
The body is almost always covered, either wholly, or in a 
great measure, by a testa, in the form of a buckler, consisting 
of a single piece in the majority, of two in some, and always 
presenting two eyes, when these organs are distinct. Two 
of their antennae ( Cheliceres , Lat.,) are, in several, shaped 
like a forceps, and perform the functions of one. The greater 
number of them have twelve feet, almost all the others ten or 
twenty -two. They live for the most part on aquatic animals, 
and most commonly on fishes. 
We shall divide this order into two families, 
The first, that of 
Xyphosura, 
Is distinguished from the following by several characters. 
There is no siphon ; the haunches of the first six pairs of feet 
are provided with small spines, and perform the office of 
jaws; the number of feet is twenty-two ; the first ten, with 
the exception of the anterior two of the males, are terminated 
in the manner of a forceps with two fingers, and inserted as 
w'ell as the follow ing two, under a large semi-lunar buckler. 
These latter support the sexual organs, and have the form of 
large leaflets, as well as the following ten, wdiich are branchial, 
and annexed to the under part of a second testa, terminated 
by a very hard, ensiform, and mobile stylet. These animals 
are, moreover, erratic. They compose the genus 
