780 CRUSTACEA. 



number of five or seven pairs ; a head, sometimes not distinct from the 

 trunk, with two or four antennas, and t^vo moveable, compound, and often 

 pedunculated eyes. The organs of generation are at the hase of the feet, 

 or at the extremity of the body. The Crustacea are, in general, to be re- 

 cognised from their solid envelope, which is sometimes extremely hard, as 

 when the calcareous matter of the covering predominates over the mem- 

 branous portion ; but, according to the families and genera, the calcareous 

 portion diminishes in quantity, and the corneous material becomes predo- 

 minant, till at last the covering seems simply membranous. 



The antenna; in this class are jointed, setaceous, and generally four in 

 number. In some, the head is intimately united to the thorax, or is indis- 

 tinguishable 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 attached below. The body is often terminated posteriorly by a tail com- 

 posed of many segments. The feet, in general, are from ten to fourteen, 

 and with six articulations, the two natural, 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 t\vo eyes, in some species elevated on moveable 

 peduncles, in others sessile. These eyes are, in general, compound or reti- 

 culated ; but in some, both eyes are united into one. The mouth is provided, 

 in general, with two mandibles, a labium below, and from three to five 

 pairs of jaws. To the first pair, or the first three pairs of these, the name 

 of the 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 branchiae are exterior in the Crustacea, although often 

 concealed, and placed at the sides, feet, or under the tail. They are, how- 

 ever, 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 ganglion 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 m.edullary 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, founded upon 

 the difference in structure and form of the branchiae, the manner in which 

 the head is connected with the trunk and the organs of mastication. The 

 first three Avere included by Linnaeus in his genus Cancer ; the fourth form- 

 ed the genus Oniscus, of that author. 



