484 



ANATOMY OF VERTEBRATES. 



themselves into the prrecavals, or directly into the great auricular 



sinus. 1 



Embryo Osseous Fish 



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 ; 2 others, teleogically, from the habits of the species. 



Five branchial 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. 3 At first the trunk of the branchial 

 arteries simply bifurcates, the divisions passing round the pharynx 



and reuniting on its dor- 

 322 sal surface, to form the 



aorta. Behind this pri- 

 mary circle, which cor- 

 responds with the fold 

 developing the hyoid and 

 mandibular arches, four 

 additional arterial hoops 

 are sent off, fig. 322, u, 

 which traverse, without 

 further ramifications, the 

 convex side of the four 

 anterior simple branchial arches, and reunite above in the aortic 

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

 ing with the fifth branchial arch, as its presence in the Lepi- 

 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 convexity 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 pullulation 

 of secondary lamina; and loops proceed, the vascular arch begins to 

 separate itself lengthwise into two channels, traversed by opposite 

 currents, and thereby establishing an arterial, fig. 318, d, and a 

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

 developements on the branchial processes. In Osseous Fishes 



1 These * venre nutritigo' are unusually large in the Carp; but are not, as Du Verney 

 supposed (cviii.), directly continued from the true ' vena? branchiales;' and they do 

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

 directly into the venous sinus. See Muller, xxi. 1841, p. 28. 



2 cxi. cxn. cxiu. 



3 The six-gilled Shark (Hexanchus) and the seven-gilled Shark (Heptanchus) are 

 among the few exceptions. 



