GILLS OF FISHES. 



4S£ 



324 



tlic primary arterial arch, corresponding with the anterior or 

 hyoid one, developes either a simple 

 (iiniscrial) gill, or a plexiform, plu- 

 mose, rudiment of a gill, or both, or 

 neither. In the Lepidostcus this arch 

 retains its primitive connection witli 

 the extremity of the branchi-arterial 

 trunk, and developes on each side a 

 small uuiserial pectinated gill, fig. 32.3, 

 1, from the membrane clothing the 

 inner surface of the cerato-hyoid and 

 preopercular bones : the vein or effe- 

 rent vessel, e, of this gill goes to a smaller 

 pectinated organ, ib. R, consisting like- 

 wise of one series of vascular filaments, whicli agrees witli the 

 ' pscudoliranchia ' of other fislies in being supjilied with arterial 

 blood. In the Sturgeon, the Lepidosiren, and tlie Plagiostomes the 

 representative of the primary vascular arch has bec(.)me, by partial 

 bifurcation of the branchi-arterial 

 trunk, a secondary branch, sent otf 

 by the artery of tlie first branchial 

 arch: but it nevertlielcss developes 

 a simple gill, of one series of filaments 

 in the Lepidosiren, fig. 324, i, and of 

 tlie anterior scries of lamella; in the first 

 gill-bag of the Plagiostomes : and this 

 series is attached, like the opercular 

 gill of tlie Lepidostcus and Sturgeon, 

 to the membrane supported by the 

 hyoid arch. 



In most Osseous Fishes we recognise 

 the reduced homologue of the anterior 

 primary vascular arch in that vessel, 

 fig. 321, e, which is continued from the 

 venous or refluent division of the 

 second jii'imary vascular arch ; not, as 

 in the foregoing fishes, from tlie ar- 

 terial division of that arch, or from 

 tlie liranchial trunk. Tlie vessel in 

 question carries, therefore, arterial 

 blood : it manifests its primitive 

 character by returning into the circulus aorticus, as at <■ , fig. 

 321, but now receives blood from it, and is called ' arteria 



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