Of the two groups into which the Ganoidei may be divided it is clear that certain members of the one (Telcostoidei), viz. Lepidosteus and Amia, shew approximations to the Teleostei, which no doubt originated from the Ganoids; while the other (Selachoidei or Sturiones) is more nearly related to the Dipnoi. Polypterus has also marked affinities in this direction, e.g. the external gills of the larva (vide p. 118).
The Teleostei, which have in common a meroblastic segmentation, had probably a Ganoid ancestor, the ova of which were provided with a large amount of food-yolk. In most existing Teleostei, the ovum has become again reduced in size, but the meroblastic segmentation has been preserved. It is quite possible that Amia may also be a descendant of the Ganoid ancestor of the Teleostei; but Lepidosteus, as shewn by its complete segmentation, is clearly not so.
The Dipnoi as well as all the higher Vertebrata are descendants of the Proto-ganoidei.
The character of the limbs of higher Vertebrata indicates that there was an ancestral group, which may be called the Proto-pentadactyloidei, in which the pentadactyle limb became established; and that to this group the common ancestor of the Amphibia and Amniota belonged.
It is possible that the Plesiosauri and Ichthyosauri of Mesozoic times may have been more nearly related to this group than either to the Amniota or the Amphibia. The Proto-pentadactyloidei were probably much more closely related to the Amphibia than to the Amniota. They certainly must have been capable of living in water as well as on land, and had of course persistent branchial clefts. It is also fairly certain that they were not provided with large-yolked ova, otherwise the mode of formation of the layers in Amphibia could not be easily explained.
The Mammalia and Sauropsida are probably independent offshoots from a common stem which may be called the Protoamniota.
(249) F. M. Balfour. A Monograph on the development of Elasmobranch Fishes. London, 1878.
(250) A. Dohrn. Der Ursprung d. Wirbelthiere und d. Princip. d. Functionswechsel. Leipzig, 1875.
(251) E. Haeckel. Schöpfungsgeschichte. Leipzig. Vide also Translation. The History of Creation. King and Co., London, 1876.
(252) E. Haeckel. Anthropogenie. Leipzig. Vide also Translation. Anthropogeny. Kegan Paul and Co., London, 1878.
(253) A. Kowalevsky. “Entwicklungsgeschichte d. Amphioxus lanceolatus.” Mém. Acad. d. Scien. St Pétersbourg, Ser. VII. Tom. XI. 1867, and Archiv f. mikr. Anat., Vol. XIII. 1877.
(254) A. Kowalevsky. “Weitere Stud. üb. d. Entwick. d. einfachen Ascidien.” Archiv f. mikr. Anat., Vol. VII. 1871.
(255) C. Semper. “Die Stammesverwandschaft d. Wirbelthiere u. Wirbellosen.” Arbeit. a. d. zool.-zoot. Instit. Würzburg, Vol. II. 1875.
(256) C. Semper. “Die Verwandschaftbeziehungen d. gegliederten Thiere.” Arbeit. a. d. zool.-zoot. Instit. Würzburg, Vol. III. 1876-1877.
[100] Monograph on the development of Elasmobranch Fishes, pp. 170-173.
[101] Hubrecht, “Zur Anat. u. Phys. d. Nervensystems der Nemertinen.” Kön. Akad. Wiss. Amsterdam; and “Researches on the Nervous System of Nemertines.” Quart. Journ. of Micr. Science, 1880.