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Show 660 PROF. HUXLEY ON THE [DeC. 14, next lower stage of vertebrate evolution ; and it is extremely interesting to observe that even the existing Amphibia present us with almost every degree of modification of the type, from such forms as the oviparous, branchiate, small-luuged Siredon and Menobranchus, which stand in the same relation to it as Gymnura to the Eutheria, to the exclusively air-breathing Salamanders and Frogs, in which the period of intraovular development, either within the uterus itself or in special receptacles, may be as much prolonged as it is in the Mammalia. A careful study, on full materials, of the development of the young of such forms as Hylodes will probably throw great light on the nature of the changes which ended in the suppression of the branchiae, and the development of the amnion and of the extra-abdominal part of the allantois in the foetus of the higher Vertebrata. The recent researches of Boas1 on the structure of the heart and the origin of the pulmonary arteries of Ceratodus fell into m y hands when I happened to be working afresh at the subject, and had arrived, so far as the heart is concerned, at results which are entirely confirmatory of his. This wonderful creature seems contrived for the illustration of the doctrine of Evolution. Equally good arguments might be adduced for the assertion that it is an amphibian or a fish, or both, or neither-the reason of this being that, as it appears to me, Ceratodus is an extraordinarily little-modified representative of that particular stage of vertebrate evolution of which both the typical Fishes and the typical Amphibia are special modifications. I think it will be convenient to have a name for the representatives of this stage ; and I propose that of Herpetichthyes. If we were to take away from Ceratodus the membrane-bones of the head and the pneumatoccele, and slightly simplify the structure of the heart, the result would be an animal which would undoubtedly be classed among the Chimeroidei; and if, in such a Chimseroid, the lamellar septa of the branchiae were not reduced, as they are in the ChiniEeroids, while the opercular fold remained undeveloped, the product would be a little-modified representative of the Selachian group, to which, among actually known forms, Heptanchus and Cestracion present the nearest approximations. Vertebrated animals in this stage of evolution may be termed Chondrichthyes. Suppose the limbs and the genital ducts of the Chondrichthyan stage to be undeveloped, and let the two nasal sacs be represented by a partially divided sac with a single external aperture, the result will be a still lower grade of vertebrate life, which may be termed Myzichthyes, represented only by the greatly modified Lampreys and Hags of the existing fauna. Finally, let the head retain its primitive segmentation, and the heart its primitive character of a contractile tube, and we reach, in the Hypichthyes, a stage of simplification of the vertebrate type, from which it would be difficult to remove any essential feature without reaching a point at which it is questionable whether an J Ueber Hcrz und Arterienbogen bei Ceratodus und Protopterus," Morph. Jahrbuch, 1880, |