156 CRUSTACEA. 



tacea they are annexed, in the shape of tufts, to five pairs of paddles 

 (feet) placed under the post-abdomen. The under part of this pos- 

 terior portion of the body is similarly furnished, in the others, with 

 four or five pairs of bifid appendages. 



ORDER I. 



DECAPODA. 



The head, in the Decapoda, is closely joined to the thorax, and 

 covered with it by a shell, entirely continuous, but that most fre- 

 quently exhibits deep lines dividing it into various regions which 

 indicates the places occupied by the principal external organs*. The 

 mode of their circulation presents characters which distinguish them 

 from the other Crustacea. The circumscribed heart f , of an oval 

 form and with muscular parieties, gives organs to six trunks of 

 vessels, three of which are anterior, two inferior, and the sixth pos- 

 terior. Of the three anterior arteries,'the median — the ophthalmic — 

 is distributed almost exclusively to the eyes ; the two others — the 

 anfennaries — spread over the shell, the muscles of the stomach, a 

 portion of the viscera and the antennae ; the two inferior ones — the 

 hepatics — transmit blood to the liver ; the last — the sternal — is the 

 most voluminous of the three, and arises from the posterior part of 

 the body, sometimes on the right side and at others on the left ; its 

 chief course is to the abdomen, and to the organs of locomotion. 

 It gives origin to a great number of large vessels, among which we 

 should particularly observe the one called by M. Audouin and Ed- 

 wards the superior ahdominal, because it arises from the posterior 

 part of that artery, at a short distance before the articulation of the 

 thorax with the abdomen, vulgarly termed the tail, and because it 

 oon dips into the abdomen — tail — where it divides into twolarge 



* M. Desmarest, in his Ilistoire Naturelle des Cmsfaces Fossilcs, and in his Cort' 

 sidtrations Generales sur la Classe des Crusfaces, has presented us, in relation to this 

 point, with an ingenious nomenclature, based on the concordunce of the portions of 

 the external surface of the shell with the organs they cover. But, in addition to the 

 fact that the shell of several Decapoda presents no impressions, or has them nearly 

 obliterated, these denominations may be replaced by others more simple, more fami- 

 liar, and relating to these same organs ; as the middle or centre, the anterior ai-.d 

 posterior extremities, the sides, &c. : it appears useless to increase our nomenclature 

 in this case. 



f These observations are extracted from the excellent memoij- of Messrs. Audouin 

 and Edwards, published in the Ann. d'Hisf. Naf., t. XI, 283 — 314, and 352 — 393. 

 See also the Mini, dii Mus. d'Hist. Nat., where M. Geofrroy Saint-Hilaire has 

 inserted the results of his curious researches ou the solids, and on the circulation of 

 the Lobster. 



