484 



ANATOMY OP VERTEBRATES. 



themselves into the prtecavals, or directly into the great aiiricnlar 

 sinus.* 



Such is the outline of the general structure of the beautiful 

 and complex mechanism of the normal or pectinated gills of fishes. 

 Of this there are many minor modifications ; some of which receive 

 explanation from known phenomena in the developement of the 

 gills ; ^ others, teleogically, from the habits of the species. 



Five brancliial arches and arteries, or vascular hoops, are 

 developed on each side in the embryo of all fishes above the Der- 

 mopteri, as a general rule.^ At first the trunk of the branchial 

 arteries simply bifurcates, the divisions passing round the pharynx 



and reuniting on its dor- 

 s' 2 sal surface, to form the 

 aorta. Behind this pri- 

 mary circle, which cor- 

 resi)onds with the fold 

 developing tlie liyoid and 

 mandibular arches, four 

 additional arterial hoops 



are sent off, fig. 



;'22 



Eiuln-j'o Osseous Fi^li 



■I, n, 

 wliicli traverse, without 

 further ramifications, the 

 couA'cx side of the four 

 anterior simple Ijranchial arches, and reunite above in the aortic 

 trunk, ib. in. If a sixth arterial arch be developed, correspond- 

 ing with the fitth branchial arch, as its presence in the Le2n- 

 dosiren would indicate, it has not been observed, and must 

 soon disappear in most Osseous Fishes. In these the gills make 

 their appearance as leaflets budding out from the con\exity of 

 the four anterior branchial arches, each leaflet supporting a 

 corresponding loop of the branchial artery ; and, as the bifur- 

 cation and extension of the primary leaflets and the pulhilation 

 of secondary laminje and loops proceed, the vascvilar arch Ijcgins to 

 scj)arate itself lengthwise into two channels, traversed by ojiposite 

 currents, and therel)y establishing an arterial, fig. 318, d, and a 

 venous, ib. c, trunk in relation to the loops and their vascular 

 developcments on tlie branchial processes. In Osseous Fishes 



' These ' YCiitc nutriiiii!' arc uiuisiuilly largo in the Carp; but are not, as l)u Vcrncy 

 suiiposed (cviu.), <lirectly eoiitinued from the true ' \cn:e braiichiales;' iiuil they Jo 

 not, therefore, divert any of the stream of arierialised blood from the aorta to pour it 

 directly into the venous sinus. Sec Midler, xxr. 1S41, ji. 28. 



^ CXI. exit. oxui. 



■' The .six-giUcd Shark (//(mkhcAw.v) and the seven-gilled Shark (l/,j<laiicl(iis) are 

 among the few exceptions. 



