GILLS OF FISHES. 



485 



323 



Branchiae and pseudo-brancnia, 

 Lepidosteus. xxii. 



324 



the primary arterial arch, corresponding with the anterior or 

 hyoid one, developes either a simple 

 (uniserial) gill, or a plexiforra, plu- 

 mose, rudiment of a gill, or both, or 

 neither. In the Lepidosteus this arch 

 retains its primitive connection with 

 the extremity of the branchi-arterial 

 trunk, and developes on each side a 

 small uniserial pectinated gill, fig. 323, 

 l, 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, which agrees with the 

 ' pseudobranchia ' of other fishes in being supplied with arterial 

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

 representative of the primary vascular arch has become, by partial 

 bifurcation of the branchi-arterial 

 trunk, a secondary branch, sent off 

 by the artery of the first branchial 

 arch : but it nevertheless developes 

 a simple gill, of one series of filaments 

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

 the anterior series of lamelhe in the first 

 gill-bag of the Plagiostomes : and this 

 series is attached, like the opercular 

 gill of the Lepidosteus 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 primary vascular arch ; not, as 

 in the foregoing fishes, from the ar- 

 terial division of that arch, or from 

 the branchial trunk. The vessel in 

 question carries, therefore, arterial 

 blood : it manifests its primitive 

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

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



Respiratory and circulatory organs, 

 Lepidosiren annectens. xxxni. 



