CRUSTACEA. 



781 



in front of the mouth, that the water reaches 

 the interior of the branchial cavity. 



Fig. 429. 



Mouth of the Leucosia. 

 Fig. 430. 



The same, without the external or posterior max- 

 illipedes. 



The branchiae contained in the two cavities, 

 one on either side, whose conformation we have 

 now described, are disposed along the vaults of 

 the flancs. They are shaped like a quadran- 

 gular pyramid, the base being fixed by means 

 of a peduncle to the inferior part of this vault 

 or to the membrane which extends from its in- 

 ferior edge to the basilar articulation of the 

 corresponding limb; some of them are even 

 inserted into this articulation. Each of these 

 organs consists of two large longitudinal vessels 

 situated on the opposite 

 edges of a transverse 

 septum, which extends 

 from the base to the 

 apex of the branchia, and 

 presents on each side 

 a great number of lamel- 

 lar or cylindrical pro- 

 longations. Of these 

 two principal vessels 

 the external is the affe- 

 rent one, of which men- 

 tion has already been 

 made in treating of the 



circulation and its organs ; the internal again is 

 the efferent vessel; the capillaries by which 

 these two communicate run in the substance of 

 the branchial lamellae, situated on either side 

 of the median septum. 



In the whole of the Decapoda brachyura and 

 anomoura,and in the greater number ofthema- 

 croura, the folds of the tegumentary membrane 

 which constitutes each branchia, are in the 

 form of very thin lamellae, directed perpendi- 

 cularly to the axis of the pyramid, and lying 

 one over another like the leaves of a book. 

 But in Crawfish, the Lobster, the Nethrops, 

 the Palinuri, the Scyllari, and the Gebiae, 

 these lamellae are replaced by a multitude of 

 small cylinders, attached by their base, and 

 closely packed side by side, like the bristles of 

 a brush. 



The number of branchial pyramids varies 



greatly, especially in the Macroura ; at the 

 most it is twenty-two, as is the case in the 

 Astacus and the most nearly allied species; 

 in other macroura the number is eighteen, as in 

 the Palinuri, Scyllari, Peneae; fifteen, as in 

 the Gebiae ; twelve, as in the Pandalus ; ten, as 

 m the Calianassae ; eight, as in the Palemons ; 

 and even seven only, as in the Crangons, Hip- 

 politi, Sergestes, &c. In the Anomoura the 

 number also varies very much. In the Bra- 

 chyura we can almost always reckon nine 

 branchiae on each side of the body; two of 

 this number, however, being merely rudiment- 

 ary; sometimes two or one of these last is 

 entirely wanting; and there are even species 

 in which the branchia, which usually occu- 

 pies the antipenultimate ring of the thorax, 

 is missing. 



The mode in which these organs are placed 

 varies in a like degree : in the Brachyura (fig. 

 426) the whole, with the exception of two rudi- 

 mentary branchiae, are arranged along one and 

 the same line, and rest parallel to one another 

 upon the vault of the flancs ; the two last rings 

 of the thorax never support any, and of the two 

 rings which correspond to the second and third 

 pairs of extremities, each presents a single py- 

 ramid attached to a hole pierced in the 

 epimeral piece near to its inferior edge (/zg.384). 

 The five branchiae, situated in front of these, 

 are attached above the edge of the vault of 

 the flancs, and with the exception of the 

 first are connected two and two upon com- 

 mon peduncles. Lastly, the two rudimentary 

 branchiae which complete the series anteriorly, 



are arranged under the base of the preceding, 

 and attached to the basilar articulation of the 

 second and third pairs of maxillary extremi- 

 ties. In the Anomoura and the Macroura, 

 the branchiae are often found arranged in several 

 ranks, and generally occur on the two last 

 thoracic segments, as well as upon those that 

 precede these (fig. 431). 



In the greater number of the Decapoda the 

 flabelliform appendages of the maxillary or of 

 the ambulatory extremities penetrate into the 

 respiratory cavity, and by their motions sweep, 

 as it were, or stroke the surface of the branchiae. 

 Some anatomists have even imagined that it was 

 by their action that the water necessary to respi- 

 ration was renewed in the interior of the branchial 

 cavities ;* but this is a mistake ; these appen- 



* Cuvier, Le9ons, t, iv. p. 432. 



