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Show 138 PROF. HUXLEY ON THE OVIDUCTS OF OSMERUS. [Mar. 20, racters of the female reproductive organs can lend no support to any attempt to draw a sharp line of demarcation between the Ganoids and the Teleosteans. , Boas1 has recently conclusively shown that the same is true of the supposed distinctive character afforded by the conus arteriosus; and it has long been admitted that the spiral valve which has been described in the intestine of Chirocentrus2 is the homologue of that which exists in all the Ganoids, though greatly reduced in Lepidosteus. Indeed I am inclined to believe that the circular valve which separates the colon from the rectum in the Smelt is merely a last remainder of the spiral valve. Thus, among the supposed absolute distinctions between tbe Ganoids and the Teleostei, only the peculiarities of the brain, and especially the so-called chiasma of the optic nerves, remain for consideration. M y lamented friend Mr. Balfour, in the last of his many valuable labours, proved conclusively that the brain of Lepidosteus is, both in structure and development, a Teleostean brain. But it is singular that no one, so far as I know, has insisted upon the fact, not only that the Teleostean brain is essentially similar to that of the Ganoids, but that it is exactly in those respects in which the Ganoids and Teleostei agree in cerebral structure that they differ most markedly from the Pla-giostomi and the Chimseroidei. In a communication read before this Society some years ago3, I pointed out that the parts of the brain termed cerebral hemispheres in the Selachians arise in a very peculiar manner, the anterior cerebral vesicle becoming subdivided by a median anterior partition, and the walls of the two ventricular cavities thus formed becoming greatly thickened. The lateral walls of the undivided part of the anterior vesicle also become thickened to form the optic thalami; but these give rise to no lobular outgrowths from their upper edges4. In the Ganoids the anterior cerebral vesicle undergoes a totally different series of modifications, inasmuch as no median septum is developed and no lateral ventricles are produced. In the Sturiones the thick lateral walls of the anterior cerebral vesicle give rise to no distinct superior lobes. In Lepidosteus, however, as Balfour has shown, such solid lobular bodies, or epithalami, are developed, and, giving rise to a thickened decurved overlapping rim from their outer faces, become exactly similar to the so-called " cerebral hemispheres" of the Teleosteans. In all the Teleosteans, in fact, the bodies called " cerebral hemispheres " are not the exact equivalents of the structures so named either in the higher Vertebrata or in the Selachians, but are 1 " Ueber den Oonus arteriosus bei Butirinus und bei anderen Knochen-fischen," Morphol. Jahrbuch, vi. 4, 1880. 2 Doubts have been thrown on the existence of this structure in Chirocentrus; so that the matter needs reinvestigation. [By the kindness of Dr. Day I have been enabled to examine a small specimen of Chirocentrus dorab; and I find it to possess just such an intestinal valve as that figured by Valenciennes. Whether it is truly "spiral" in its arrangement, or not, can only be determined by the examination of a larger specimen.-T. H . H., July, 1, 1883.] 3 " O n Ceratodusforsteri," Proc. Zool. Soc. January 4, 1876. 4 See Balfour, ' Development of the Elasmobranchs.' |
