THE LAMPREYS. 291 



Muller calls the caudal heart, renders the fish unable to use 

 the powerful muscles of the tail. 



Much has yet to be learned of the natural history of 

 this fish, particularly as to its migration, mode of propa- 

 gating its species, and the localities in which that process 

 takes place, whether only in brackish or salt water, or in 

 both fresh and salt water. Up to the present time the 

 evidence is against the fish breeding in fresh water. Much 

 more positive information is required on this particular 

 point. 



THE LAMPREYS. 



In the third sub-class, Cyclostomata, or circular-mouthed 

 fishes having a cartilaginous skeleton, without ribs or real 

 jaws, are found 



The LAMPREYS, which belong to the first family of this 

 third sub-class the Petromyzontidce characterised by the 

 body being eel-shaped and naked, and subject to meta- 

 morphosis. 



THE LAMPREY. 



Gunther ("Study of Fishes," p. 1 70) says: "Changes 

 amounting to metamorphosis have been hitherto observed 

 in Petromyzon only. In the larval condition (Ammoc'cetes) 

 the head is very small, and the toothless buccal cavity is 

 surrounded by a semi-circular upper lip ; the eyes are ex- 

 tremely small, hidden in a shallow groove, and the vertical 

 fins form a continuous fringe. In the course of three or 

 four years the teeth are developed, and the mouth changes 

 into a perfect suctorial organ ; the eyes grow, and the 

 dorsal fin is divided into two divisions." 



