. Comparative anatomy. Anatomy, Comparative. HYPOPHYSIAL DUCT pharynx/ HYPOBRANCHIAL MUSCLE (RESPIRATORY TUBE HYPOPHYSIAL ^ DUCT ^=-^. NOTOCHORO I ST GILL APERTURE B. PETROMYZON. HYPOBRANCHIAL MUSCLE Fig. 250.—Diagrams of median longitudinal sections of the heads of Bdellostoma and Petromyzon, showing the relations of the hypophysial ducts in the two forms. In the former the hypophysial duct opens posteriorly into the pharynx, suggesting the possibil- ity that it may once have served as a mouth (paleostoma). In petromyzon the hypo- physis fails to open into the pharynx and is co
. Comparative anatomy. Anatomy, Comparative. HYPOPHYSIAL DUCT pharynx/ HYPOBRANCHIAL MUSCLE (RESPIRATORY TUBE HYPOPHYSIAL ^ DUCT ^=-^. NOTOCHORO I ST GILL APERTURE B. PETROMYZON. HYPOBRANCHIAL MUSCLE Fig. 250.—Diagrams of median longitudinal sections of the heads of Bdellostoma and Petromyzon, showing the relations of the hypophysial ducts in the two forms. In the former the hypophysial duct opens posteriorly into the pharynx, suggesting the possibil- ity that it may once have served as a mouth (paleostoma). In petromyzon the hypo- physis fails to open into the pharynx and is converted into a pipette-like organ into which the olfactory pits open. On the basis of this difference cyclostomes are divided into two sub-classes, Hyperotreta and Hyperoartia. gut in vertebrate embryos suggests that the alimentary canal formerly extended anterior to the present mouth; and, finally, that in the Myxinoids and the embryonic sturgeon the hypophysis actually opens into the pharynx, and like the mouth of urochordate larvae, has a dorsal external opening. Kupffer, however, does not waste time guessing as to the cause of this substitution of a new mouth for an old one. It is possible that the development and enlargement of the pre-oral lobe was a factor in effecting the displacement of the mouth from a dorsal to a ventral position. Diagrams showing the position of the four mouths mentioned in this discussion are shown in Fig. 249. The objection to the idea that there have been a series of mouths in the course of animal phylogenesis, on the ground that the chances are against the appearance of more than one ingestive opening into the enteron, loses much of its weight in view. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Neal, Herbert V. (Herbert Vincent), 1869-1940; Rand, Herbert W. (Herbert Wi
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