MAMMALIA SAUROPSIDA | | ----------------------------- | PROTO-AMNIOTA AMPHIBIA | | ---------------------------------- | TELEOSTEI PROTO-PENTADACTYLOIDEI | | GANOIDEI | ---- DIPNOI | | --------------------| PROTO-GANOIDEI | |-- HOLOCEPHALI |------- ELASMOBRANCHII | PROTO-GNATHOSTOMATA | --------------------------| | | _Cyclostomata_ PROTO-VERTEBRATA | --------------------------|------------------------- | | _Cephalochorda_ PROTOCHORDATA _Urochorda_ In the above table the names printed in large capitals are hypothetical groups. The other groups are all in existence at the present day, but those printed in Italics are probably degenerate. The ancestral forms of the Chordata, which may be called the Protochordata, must be supposed to have had (1) a notochord as their sole axial skeleton, (2) a ventral mouth, surrounded by suctorial structures, and (3) very numerous gill-slits. Two degenerate offshoots of this stock still persist in Amphioxus (Cephalochorda), and the Ascidians (Urochorda). The direct descendants of the ancestral Chordata, were probably a group which may be called the Protovertebrata, of which there is no persisting representative. In this group, imperfect neural arches were probably present; and a ventral suctorial mouth without a mandible and maxillæ was still persistent. The branchial clefts had, however, become reduced in number, and were provided with gill-folds; and a secondary head (_vide_ p. 313), with brain and organs of sense like those of the higher Vertebrata, had become formed. The Cyclostomata are probably a degenerate offshoot of this group. With the development of the branchial bars, and the conversion of the mandibular bar into the skeleton of the jaws, we come to the Proto-gnathostomata. The nearest living representatives of this group are the Elasmobranchii, which still retain in the adult state the ventrally placed mouth. Owing to the development of food-yolk in the Elasmobranch ovum the early stages of development are to some extent abbreviated, and almost all trace of a stage with a suctorial mouth has become lost.