OEDEES OF FISHES. 



239 



Order II. Marsipobranchii. 



This order includes the Hag-fislies [Mi/xinida-) and tlie Lampreys 

 {Petromyzonido!), and it is defined by the following cliaiacters : The 

 body is cylindrical and worm-like, and is destitute nf liniljs. The 

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

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

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

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

 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 



Fi;^. 172. — Morphology of Mar-sipobrancliii. A, Mii-iini; tjh'diio^n, llir ling lisli, s]iow- 

 ing the sucker-like inoutli, and the two \'enti';il openings (/O liy wliicli Hie Wiitcr 

 escapes from the gills. B, The Ri\'er Lamprey <:ir Laniperji {r<?trom;f:oii JIuriafilh), 

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



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

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

 very peculiar e.x.istence. It is generally found embedded in the 

 interior of some large fish, into whicji 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 foiuis a sucking 

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

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

 these and the Mud-fishes {Lepidosiren and i'cratodvs), 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 



