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Even in the Dipneusta the fin is of the same construction as in the fishes; it was afterwards gradually evolved into the five-toed form, which we first encounter in the Amphibia.
Another considerable innovation is the proposition to divide the class Pisces into the four groups of Leptocardia, Cyclostomata, Pisces, and Dipneusta. And there is much to be said in favour of the same promotion of the Cyclostomata, or Lampreys and Hags.
Proselachii. : 16. Natidanides. Chlamydoselachius. Heptanchus. Silurian. 17. Ganoides. Plated-fishes. Proganoides. : 17. Accipenserides. Devonian. : 18. Dipneusta. Paladipneusta. : 18. Neodipneusta. Ceratodus. Protopterus. Carboniferous. : 19. Amphibia. Stegocephala. : 19. Phanerobranchia. Salamandrina. Permian. : 20. Reptilia. Proreptilia. : 20. Rhynchocephalia. Primitive lizards. Hatteria.
Comparative anatomy and ontogeny show clearly that the Ganoids descended from the Selachii, and the Teleostei from the Ganoids. On the other hand, a collateral line, or rather the advancing chief line of the vertebrate stem, was developed from the earlier Ganoids, and this leads us through the group of the Dipneusta to the important division of the Amphibia.
This reduction of the number of the toes to six, and then to five, probably took place in the second half of the Devonian period at the latest, in the subsequent Carboniferous period in those Dipneusta which we regard as the ancestors of the Amphibia. We have several fossil remains of five-toed Amphibia from this period. B larva, restored, with gills.
In view of the close correlation of the muscular and nervous systems, this also made great advance in structure and function. Hence we find, as a matter of fact, that the brain is much more developed in the higher Amphibia than in the fishes, the Dipneusta, and the lower Amphibia.
Hence we find in the lowest Amphibia, the gilled Amphibia, that, like the Dipneusta, they pass the greater part of their life in the water, and breathe water through gills. They only come to the surface at brief intervals, or creep on to the land, and then breathe air by their lungs.
I can note only a few points which seem to me to be open to discussion. The Monorhina, having been developed out of the Leptocardia, gave rise, according to Professor Haeckel, to a shark-like form, which was the common stock of all the Amphirhina. From this "Protamphirhine" were developed, in divergent lines, the true Sharks, Rays, and Chimaerae; the Ganoids, and the Dipneusta.
But considering the close relation of the Mudfish with the Ganoidei, and the wide differences between the Elasmobranchii and the Teleostei, I greatly doubt the propriety of separating the Dipneusta, as a class, from the other Pisces.
The true Amphibia are nearer to the Dipneusta and the fishes; they are also much older than the reptiles. It is probable that the Amphibia were evolved even earlier during the Devonian period from the Dipneusta.
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