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 (Petromyzon 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. SYSTEMATIC SURVEY _Of the 4 Main-classes, 9 Classes, and 26 Sub-classes of Vertebrata._ Gen. Morph. vol. ii. Plate VII. pp. 116-160. ====================================================================================== I. =Skull-less= (+Acrania+), or =Tube-hearted= (+Leptocardia+). Vertebrata without head, without skull and brain, without centralized heart. -------------------------------------------------------------------------------------- 1. =Skull-less= I. Tube-hearted { 1. Lancelet 1. Amphioxus +Acrania+ _Leptocardia_ { -------------------------------------------------------------------------------------- II. =Animals with skulls= (+Craniota+) and with =thick-walled hearts= (+Pachycardia+). Vertebrata with head, with skull and brain, with centralized heart. ====================================================================================== _Main-classes_ | _Classes_ | _Sub-classes_ | _Systematic Name_ _of the Skulled_ | _of the_ | _of the_ | _of the_ _Animals._ | _Skulled Animals._ | _Skulled Animals._ | _Sub-classes._ -------------------------------------------------------------------------------------- 2. =Single-Nostriled= { II. Round mouths { 2. Hags, or Mucous 2. Hyperotreta +Monorrhina+ { _Cyclostoma_ { Fish (Myxinoida) { { 3. Lampreys, or 3. Hyperoartia { { Pride (Petromyzontia)