ORGANS OF RESPIRATION. 547 



ductus pneumaticus, to the air-sac, or to aerate the vascular 

 mucous lining of their intestine. As the great artery pro- 

 ceeding from the heart of fishes is entirely subdivided into 

 branchial capillaries, and leaves no pervious anastomosing 

 trunk communicating with the veins, as in amphibia, all the 

 species are necessitated to retain permanently the aquatic 

 character of the earliest tadpole state, incapable of undergoing 

 metamorphosis by the absorption of the branchiae. The 

 trunks of the branchial veins returning the arterialized blood 

 to the descending aorta, occupy the bottom of the marginal 

 concavities of the branchial arches; the branchial arteries 

 are exterior to them in these grooves ; and in each small 

 leaflet of the gills the arterial twig occupies the inner margin, 

 and the returning vein the exterior edge. 



The number of branchial arches varies in fishes, as in amphi- 

 bia ; in some plectognathi, as the diodon, there are only three 

 pairs of gills ; there are three pairs in the lophius, with the rudi- 

 ment of an anterior fourth pair ; most osseous fishes present 

 four complete pairs ; several present the rudiment of an ante- 

 rior fifth pair ; and a greater number is seen in many of the car- 

 tilaginous fishes. In the operculated fishes, whether osseous 

 or cartilaginous, there are only two branchial openings, one on 

 each side of the pharyngeal cavity ; in the long cylindrical 

 ffastrobranchus, likewise, there are but two small round 

 branchial openings, which are the terminations of two com- 

 mon lengthened canals, which receive the whole of the branchial 

 currents, and convey them to the surface. In the sturgeon, 

 the chimaera, and the spatularia, though cartilaginous fishes, 

 the branchiae are free at their margins, with a single opercular 

 opening on each side of the neck ; in the rays and sharks, where 

 there is no free operculum, there are five narrow transverse 

 openings on each side ; and the outer edges of the branchiae 

 are fixed to the integuments between the several openings ; 

 so that the inhaled water is divided into five streams, passing 

 over the surfaces of all the separate gills. In the lampreys* also 

 with fixed branchiae and without a free operculum. there are 

 seven of those separate branchial cavities on each side,with as 

 many small round openings, through which the respiratory 

 currents pass to and fro, as in the lateral respiratory sacs of 

 a leech or a worm. In place of the usual pectinated gills 

 seen in other osseous and cartilaginous fishes, the lopho- 



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