LAMPREYS AND HAG-FISHES, 
55 1 
straight and simple; and the reproductive organs discharge into the cavity of the 
body. The place of teeth is taken in some forms by horny structures, while in 
others the mouth is completely unarmed. Some difference of opinion exists among 
naturalists as to whether the absence of the true jaws in the lampreys is an original 
or an acquired feature; but, to our mind, the apparent want of these organs in 
the primitive extinct lampreys seems to be strongly in favour of the former view. 
The true lampreys, of which the sea - lamprey (Petromyzum 
True Lampreys. . . . . r J ’ _ . . . 7 . . 
mamnus), river-lamprey (P. jluviatiLis), and the small lamprey 
(P. branchialis ) occur in Britain, are the typical representatives of a family 
SEA-LAMPREY, RIVER-LAMPREY, AND SMALL LAMPREY (J uat. Size). 
(Petromyzidce) characterised by the nasal duct terminating in a closed sac 
behind, without perforating the palate. As in all the other members of the group, 
the naked body is eel-like in form; but the family is peculiar in that its members 
undergo a metamorphosis, the young being devoid of teeth, and furnished with a 
single median fin, whereas in the adult the sucking-mouth is furnished with horny 
teeth resting on a soft cushion, and the median fin is divided. In the adult the 
tongue is furnished with rasping teeth, while above and below the aperture of the 
mouth there are a series of upper and lower teeth, and the sucking-disc is likewise 
provided with smaller isolated teeth. Eyes are present in the adult; and the 
aperture of the nostrils is situated in the middle of the head. The seven-gill 
pouches open externally by as many apertures on each side of the neck, but com- 
