THE FAMILY OF " ROUND-MOUTHED " FISH. 405 



gills, and through these apertures the water obtains access 

 and egress, or by an aperture through the top of the head 

 communicating with the throat. When the fish is swim- 

 ming free, the water also reaches the gills by a peculiar 

 membranous canal, or trachea, placed under the gullet 

 and perforated with holes. The position of the gill-cells 

 &c., is exhibited in the accompanying engraving of the 

 neck of the River-Lamprey, or Lampern as it is more 

 commonly called, a portion of the skin being removed to 

 show the arrangement beneath. 



The Lampreys are ali oviparous, and spawn late in the 

 spring. 



To return to the common Lamprey (Petromyzon ma- 

 rinus) . 



This species appears to be generally diffused over most 

 of the rivers and seas of Europe and America, attaining, 

 however, a larger size in the southern than in the northern 

 latitudes in the colder climates seldom exceeding from 

 17 to 20 inches. During spring and summer they abound 

 to a considerable extent in some of the streams on the 

 southern coast of England, particularly in the Severn, and 



