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 Lamperns, or Nine-eyes, of our rivers (Petro- 
myzon fluviatilis), 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 
