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 pai'ts 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 

 well as the following 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, which 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 



