CRUSTACEA. 145 



calcareous portion diminishes in quantity, and the corneous ma- 

 terial becomes predominant, till at last the covering seems simply 

 membranous. 



The antennae in this class are jointed, setaceous, and gene- 

 rally four in number. In some the head is intimately united 

 to the thorax, or is indistinguishable as a separate organ. The 

 shield in this case forms a large covering over the thorax, which 

 is called the shell. In others, where the head is distinct, the 

 body is divided into seven segments, to which the feet are at- 

 tached below. The body is often terminated posteriorly by a 

 tail composed of many segments. The feet in general are from 

 ten to fourteen, and with six articulations, the two anterior feet, 

 and sometimes the two or four following ones, being terminated in 

 a kind of forceps ; at other times in simple hooks ; and in some 

 the termination of the feet seems adapted only for swimming. 



The Crustacea have two eyes,m some species elevated on move- 

 able peduncles, in others sessile. These eyes are in general 

 compound or reticulated ; but in some both eyes are united in- 

 to one. 



The mouth is provided in general with two mandibles, a la- 

 Hum below, and from three to five pairs of jaws, To the first 

 pair, or to the first three pairs of these, the name of feet-jaws has 

 been applied, as being formed by the two or six anterior feet of 

 the animal, modified by their position near the mouth, and not 

 proper for locomotion. 



The branchice are exterior in the Crustacea, although often 

 concealed, and placed at the sides, feet, or under the tail. 

 They are, however, generally at the base of one part of the 

 feet, and are composed of pyramidal laminae, or tufted filaments. 



The nervous system in this class is very similar to that of 

 the Arachnides and Insects. It consists principally of a gan- 

 glion or brain, placed before and above the intestinal canal, and 

 of an elongated double chord with ganglions or knots placed on 

 the lower surface of the body, sometimes extending its whole 

 length, at others forming towards the middle a medullary circle, 

 with radiated elongations. The Crustacea enjoy the faculty of 

 sight ; many of them that of hearing ; and they possess the 

 senses of smell and taste ; but their sense of touch is obscure, 

 from their calcareous or horny covering. 



The class Crustacea is divided by Latreille into five orders, 



VOL. II. K 



