THE LAMPREYS AND HAGS. 203 



separating the Monorrhina, as we have separated the Skull- 

 less animals, from the Fishes, with which they have hitherto 

 been erroneously classed. 



We owe our first accurate knowledge of the Monorrhina, 

 or Cyclostoma, to the great zoologist, Johannes Miiller of 

 Berlin ; his classical work on the " Comparative Anatomy 

 of the Myxinoida" forms the foundation of our modern 

 views on the structure of the Vertebrate animals. He 

 distinguished two distinct groups among the Cyclostoma, 

 which we shall consider as sub-classes. 



The first sub-class consists of the Hags (Hyperotreta, or 

 Myxinoida). They live in the sea as parasites upon other 

 fish, into whose skin they penetrate (Myxine, Bdellostoma). 

 Their organ of hearing has only one annular canal, and 

 their single nasal tube penetrates the palate. The second 

 sub-class, that of Lampreys, or Prides (Hyperoartia, or 

 Petromyzontia) is more highly developed. It includes the 

 well-known Lampems, or Nine-eyes, of our rivers (Petro- 

 myzon fluviatihs), with which most persons are acquainted. 

 They are represented in the sea by the frequently larger 

 marine or genuine Lampreys (Petromyzon marinus). The 

 nasal tube of these single-nostriled animals does not 

 penetrate the palate, and in the auricular organ there are 

 two annular canals. 



All existing Vertebrate animals, with the exception of 

 the Monorrhina and Amphioxus just mentioned, belong to 

 the group which we designate as Double-nostriled animals 

 (Amphirrhina). All these animals possess (in spite of the 

 great variety in the rest of their forms) a nose consisting of 

 two lateral halves, a jaw-skeleton, a sympathetic nervous 

 system, three annular canals connected with the auricular 



