2928 



LOWEST VERTEBRATES AND THEIR ALLIES 



by horny structures, while in others the mouth is completely unarmed. Some dif- 

 ference 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 favor 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 {Petromy- 

 zidce} characterized by the nasal duct terminating in a closed sac behind, without 



True Lam 

 preys 



SEA LAMPREY, RIVER LAMPREY, AND SMALL LAMPREY. 

 (One-fourth natural size.) 



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 meta- 

 morphosis, 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 w r ith 

 smaller isolated teeth. Eyes are present in the adult; and the aperture of the nos- 



