THE PHYLOGENESIS OF VERTEBRATES 29 



as teeth. A most important distinction between Amphio.rus 

 and the cyclostomes, however, lies in the fact that the latter 

 possess a definite head, with brain and sense organs, parts 

 which exist only in a rudimentary or potential sense in Am- 



phlOJCUS. 



In distinction from the cyclostomes, or " round-mouths," 

 are the true vertebrates, which are termed gnathostomes, or 

 " jaw-mouths," the possession of jaws being a constant char- 

 acteristic of the entire group. The lowest class of gnathos- 

 tomes is that of the fishes (Pisces), but these are in turn 

 subdivided into several groups, some of which represent 

 lateral branches, that is, specializations along definite direc- 

 tions, and thus not in the direct line of human history. The 

 most primitive group is that of selachians, which comprises 

 the sharks and dog-fish, and the skates or rays. This group 

 of animals is absolutely fundamental for the morphologist 

 and represents the first great stage in the main line of verte- 

 brate history. Selachians have a wholly cartilaginous skele- 

 ton, the mouth upon the lower side of the head and not at the 

 anterior end, as in other fish, and five gill-slits which open 

 separately and free, not covered by an operculum (gill-flap). 

 Their position in the tree is clearly in the main line above 

 the cyclostomes. 



The ganoid fishes are also of great importance to us. 

 They represent a T:ew remnants of what was the dominant 

 group during the Devonian epoch and are the direct de- 

 scendants of the selachians. As in the case of all such rem- 

 nants, they are extremely diverse in structure among them- 

 selves and are placed in a single group rather more for con- 

 venience than because of a very close relationship to one 

 another. They are characterized by the tendency of the 

 scales to fuse into bony plates, a tendency which in the past 

 resulted in the development of a special group, -the placo- 

 dcrms, which were entirely covered by a suit of mail formed 

 in this way. Similar plates cover the head in all modern 

 ganoids and they occur in rows along the body in a few 

 forms (sturgeons). The skeleton is mainly cartilaginous in 



