102 THE EVOLUTION OF MAN. 



(Cyclostoma), from the fact that their mouth forms a circular 

 or semi-circular opening. The upper and under jaws, 

 which appear in all the higher Vertebrates, are completely 

 wanting in the Kound-mouths, as in the Amphioxus. All 

 other Vertebrates are therefore distinguishable from them 

 as "Jaw-mouthed" {Gnathostomi). The Round-mouths may 

 also be called " Single-nostrils " {Monorhina), because they 

 have but a single nasal tube, while the Gnathostomi are all 

 furnished with a pair of nasal cavities, a right and a left 

 nose-cavity f' Double-nostrilled," Amphirhina). But in 

 addition to these peculiarities, the Jaw-mouths are also 

 distinguished by many other remarkable structural arrange- 

 ments, and are further removed from the Fishes than the 

 latter are from Man. They must, therefoi'e, evidently be 

 regarded as the last remnant of a very old and very low 

 class of Vertebrates, which are far below the structural 

 stage of a genuine Fish. To mention here briefly only 

 the most important, the Round-mouths are entirely with- 

 out any trace of limbs. Their slimy skin is quite 

 naked and smooth, without scales. They are wholly 

 destitute of a bony skeleton. The inner skeleton axis is 

 a very simple inarticulate notochord, like that of the 

 Amphioxus. In the Lampreys alone a rudimentary articu- 

 lation is indicated by the fact that upper arches appear in 

 the vertebral tube proceeding from the notochord sheath. 

 At the anterior end of the chorda a skull is developed in 

 its very simplest form. From the notochord sheath pro- 

 ceeds a small soft-membraneous skull capsule, which 

 becomes partly cartilaginous: this capsule encloses the 

 brain. The important apparatus of the gill-arches, the 

 tongue-bone, etc., which is inherited by all Vertebrates 



