TELEOSTEI. 357 



of the single gill-slit, covered by a gill-cover, as seen in the 

 ordinary bony fishes, the side of the neck, when viewed ex- 

 ternally, exhibits six or seven round holes placed far back in a 

 line on each side (fig. 133, A). In the Hag-fishes the external 

 apertures of the gills are reduced to one on each side, placed 

 below the head ; but the internal structure of the gills is the 

 same in both cases. In both the Lampreys and the Hag-fishes, 

 namely, the gills are in the form of sacs or pouches (fig. 133,6), 

 the mucous membrane of which is thrown into folds or plaits 

 like the leaves of a book, over which the branchial vessels 

 ramify. Internally the sacs communicate with the cavity of 

 the pharynx, either directly or by the intervention of a common 

 respiratory tube. It follows from this, that the gill-pouches on 

 the two sides, with their included fixed branchial laminae, com- 

 municate freely with one another through the pharynx. The 

 object of this arrangement appears to be mainly that of ob- 

 viating the necessity of admitting water to the gills through the 

 mouth, as is the case with the ordinary bony fishes. These 

 fishes are in the habit of fixing themselves to foreign objects 

 by means of the suctorial mouth ; and when in this position it 

 is, of course, impossible that they can obtain the necessary 

 water of respiration through the mouth. As the branchial 

 pouches, however, on the two sides of the neck communicate 

 freely with one another through the pharynx, water can readily 

 pass in and out. This, in the Lampreys, is further assisted 

 by a kind of elastic cartilaginous framework upon which the 

 respiratory apparatus is supported, and which acts somewhat 

 like the ribs of the higher Vertebrata. Water can also be 

 admitted to the pharynx, and thence to the branchial sacs, by 

 means of a tube which leads from the pharynx to an aperture 

 placed on the top of the head. 



The Lampreys are, some of them, inhabitants of rivers ; but 

 the great Sea-lamprey (Petromyzon marinus] only quits the 

 salt water in order to spawn. The Hag-fish ( Myxine glutinosa) 

 is an inhabitant of the North Sea, and is commonly captured 

 on the Norwegian coast. 



CHAPTER LVI. 

 TELEOSTEI. 



ORDER III. TELEOSTEI. This order includes the great ma- 

 jority of fishes in which there is a well-ossified endoskeleton, 



