LAMPREYS AND HAG-FISHES. 551 



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 

 marinus), river-lamprey (P. fluviatilis), and the small lamprey 

 (P. branchialis) occur in Britain, are the typical representatives of a family 



True Lampreys. 



SEA-LAMPREY, RIVER-LAMPREY, AND SMALL LAMPREY ( liat. 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 ths 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- 



