142 CRUSTACEA. 



twelve feet(l), 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(2). 



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 covered with small spines and perform the 

 office of jaws ; there are twenty-two feet ; the first ten, with 

 the exception of the two anterior ones in the males, are ter- 

 minated by a didactyle forceps, and inserted, as well as the 

 two that follow, under a large semi-lunar shield ; the latter 

 have the sexual organs attached to them, and the form of large 

 leaflets, as in the case with the ten following, which are bran- 

 chial and inserted under a second shell, terminated by a very 

 hard, ensiform and movable stylet. They are wandering ani- 

 mals, and form the genus 



Limulus, Fab. 



The species are known in commerce by the name of the Molucca 

 Crab. The suborbicular, slightly elongated and posteriorly narrow- 

 ed body is divided into two parts, invested by a solid shell com- 

 posed of two pieces, one to each part, very hollow beneath, and pre- 

 senting above, two longitudinal sulci, one on each side, and a carina 

 on the middle of the back. The first part of the shell, or that which 

 covers the fore-part of the body, is much larger than the other, forms 

 an extensive semi-lunar shield, with a reflected edge, furnished above 

 with two oval eyes of numerous facets, resembling granules, one on 

 each side, exterior to a longitudinal carina; and on the anterior ex- 



(1) Fourteen in several, according 1 to Leach; those which he considers as the 

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

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

 twelve feet. 



(2) In my Fam. Nat. du Regne Anim., they form two orders. 



