404 MANUAL OF ZOOLOGY. 



which is in reality an unpaired nostril. In the Lampreys, on 

 the other hand, the single nasal sac has the same structure as 

 in the typical fishes that is to say, it is closed behind, and 

 does not communicate in any way with the cavity of the mouth. 



Another very remarkable point in the Hag-fishes and Lam- 

 preys is to be found in the structure of the gills, from which 

 the name of the order is derived. In the Lampreys, in place 

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

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

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

 line on each side (fig. 170, 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. 170, B), 

 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, 

 communicate freely with one another through the pharynx. 

 The object of this arrangement appears to be mainly that of 

 obviating 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 mouth in the Petromy- 

 zonida is a circular cartilaginous ring, formed by the amalga- 

 mation of the palatine and mandibular arches, and carrying 

 numerous teeth and small tubercles. The tongue is armed 

 with a double series of small teeth, and acts like a piston, 



