ORDERS OF FISHES. 



239 



Ordee II. Maesipobranchii. 



This order includes the Hag-fishes [MyxinicUe) and the Lampreys 

 {Petronxyzonidm), and it is defined by the following characters : The 

 body is cylindrical and worm-like, and is destitute of limbs. The 

 skull is cartilaginous, there is no lower jaw, and the notocliord re- 

 mains through life. The month is circular and cup-like. The heart 

 is composed of an auricle and a ventricle, but there is no hulbus 

 arteriosus. The gills are pouch-like, communicating with the throat 

 on the one hand, and opening externally on the other by means of 

 apertures placed on the sides of the neck, or on the ventral surface. 



The Hag-fish (Myxine) is an eel-like fish (fig. 172, A), which 

 agrees with the Lampreys in having neither pectoral nor ventral 



Fig. 172. — Morphology of Marsipobranchii. A, Myxine glutinosa, the Hag-fish, show- 

 ing the sucker -like moutli, and the two ventral openings (ft) by which the water 

 escapes from the gills. B, The River Lamprey or Lanipern (Petromyzoii fuviatiiis), 

 showing the seven branchial apertures on the side of the neck. 



fins, the representatives of the fore and hind himbs. The mouth is 

 of a very remarkable character, and enables the Hag-fish to lead a 

 very peculiar existence. It is generally found embedded in the 

 interior of some large fish, into which it has penetrated by means 

 of a single serrated and recurved fang attached to the centre of the 

 palate. The mouth itself is destitute of jaws, and forma a sucking 

 disc or cup. Another remarkable peculiarity of the Hag-fishes is 

 found in the structure of the nose. In all fishes, namely, except 

 these and the Mud-fishes {Lepidosiren and Ceratodus), the nasal 

 chambers are closed behind, and do not communicate with the 

 cavity of the mouth, as they do in the higher Vertebrates. In the 



