444 CRUSTACEA. 



THE SECOND ORDER OF ENTOMOSTRACA,— 



{The Seventh and last of the Class Crustacea), — 



PCECILOPODA,— 



Is distinguished from the preceding by the diversity in the form of the feet, of 

 which the anterior, of an indeterminate number, are ambulatory, or fitted for jDre- 

 hension, and the others, lamelUform or pinnated, are branchial, and fitted for swim- 

 ming. But it is especially in the absence of mandibles and maxillae of the ordinary 

 form that they are separated from all the other Crustacea ; sometimes these organs 

 are replaced by the basal joint of the six anterior legs being armed with numerous 

 minute spines ; sometimes the organs of manducation consist either in an external 

 siphon in the form of an inarticulated beak, or in some other instrument fit for 

 suction, but hidden, or very indistinct. 



The body is nearly always covered, either entirely or for the most part, by a 

 shell in the •shape of a shield, composed of a single piece in the majority, but of 

 two parts in some, and always exhibiting two eyes at least when these organs are 

 distinct. Two of the antennse (Chelicerce, Latr.) are in many in form of hooks, 

 and perform the functions as such. The number of their legs is twelve in the 

 greater number*, and of ten or twenty-two in nearly all the others. They reside 

 ■for the most part upon aquatic animals, and most commonly on fishes. 



We divide this Order into two families, [Xyphosura and Siphonostoma,] which, in 

 my Families Naturelles, composed two separate orders. 



THE FIRST FAMILY OF PCECILOPODA,— 

 Xyphosura, — 

 Is distinguished from the following by many characters : they have no siphon ; the coxae of the six pairs 

 of fore-legs are beset with minute teeth, and perform the office of jaws ; the number of legs is twenty- 

 two ; the ten anterior, ^^•ith the exception of the two anterior in the males, are terminated by a two- 

 fingered claw, and inserted, as well as the two following, beneath a large semilunar shield ; the latter 

 bear the organs of generation, and are in the form of large leaves, as well as the ten following, which 

 are branchial, and annexed to the under side of a second shield, which is terminated by a veiy powerful, 

 horny, moveable style, like a sword. These animals are wanderers. They compose the genus 



Limulus, Fab., of which the species have received the name of [King Crabs], or crabs of the JMohiccas. The 



nearly rounded body, somewhat elong^ated and narrowed behind, is divided into two parts, and covered by a solid 



shell of two pieces, one for each division of the body ; it is very concave beneath, and exhibits 



on its upper side two long'itudinal impressions, one on each side, and a central dorsal ridge. 



Tlie fore part of the shell, or that which covers the front of the body, is much larger than the 



other, and forms a large semilunar shield, having on its upper side two oval eyes, with very 



nimftrous facets, in the form of minute grains, and situated one on each side on the outside of 



the longitudinal ridge ; and at the anterior extremity of that of the centre, which extends to the 



pieces of the shell, are two small, simple eyes, close together. Within the cavity of the anterior 



shell is a small swollen labrum, ridged in the centre, terminated in a point, and above which 



are inserted two small antennse, in the form of small didactyle claws, and elbowed in the middle 



of their length, at the union of the first and following joint. Immediately beneath are inserted, 



close together in pairs, in two lines, twelve legs, of which the ten anterior (the two or four anterior 



in the males only excepted) are terminated by a didactyle claw, and of which the basal joint is 



Fig. 23.— Limulus advanced interiorly into a lobe armed with numerous minute spines, and performs the functions 



of the maxillae. These legs progressively increase in size, and, with the exception of the fifth 



pair, are composed of six joints, including the moveable finger of the claw ; the fifth pair have an additional 



joint, and also a curved appendage at the base, directed backwards, and composed of two joints ; their fifth 



* Fourteen in some siiecics, ai-cordiiio; 10 Leach ; bul the pair wliich , fcrior anteiiiiiie. The ArguU, which, in respect tn their locomotive 

 he considers to be the anterior puir, appcurs to me to be the two in- ! oryaus, are the most perfect, have only twelve lei's. 



