based upon the different position of the communication between them and the alimentary canal.
But another and perhaps more weighty objection has been urged by Prof. Huxley. He says: "But such air-sacs are air-bladders and not lungs, because they receive their blood from the adjacent arteries of the body, and not direct from the heart, while their efferent vessels are connected only with the veins of the general circulation."
According to this view, therefore, the Dipnoans (Protopterus and Lepidosiren) have lungs, because the blood goes to the air-sacs by a pulmonary artery, and returns by a pulmonary vein into a left auricle; while the cellular and vascular air-bladders of Amia and Lepdosteus are not lungs, because such an arrangement does not exist.
Yet Prof Huxley applies the name placenta to the vascular inter-digitations by which the young of some sharks are connected with the mother, although they are developed from the yolk and not, as in mammals, from the chorion. It would be interesting to know whether the nerves of the air-bladder are the same as those of the lungs.
The best test of the naturalness of the definition would be furnished by the discovery of some form having the pulmonary vessels connected with an air-bladder lying upon the dorsal side of the alimentary canal. Meantime, since all are agreed upon the facts, the question concerns interpretations and definitions.
Whether or not the air-bladder of the gar-pike is entitled to the name of lung, we may admit that it corresponds with a lung in its essential connection with the alimentary canal, and apparently in its function as an organ for aiding the oxygenation of the blood.
THE aphorism that "history repeats itself" is in no case more true than in regard to the subject on which I am now to address you. For there has been a continuity from the very earliest times of a belief, more or less general, in the existence of "occult" agencies, capable of manifesting themselves in the production of mysterious phenomena,
- In fact, considering the resemblance of the brains and enameled scales of Lepidosteus and Polypterus, and the differences of their air-bladders and ducts, one is inclined to regard the latter as of slight taxonomic importance.
- This discussion, in which the subjects are considered historically and scientifically, is an expansion of the lectures delivered at the London Institution.