. Amphioxus and the ancestry of the vertebrates. Amphioxus; Sea squirts; Hemichordata. INTERNAL ANATOMY. 23 frog-tadpole the fold of one side becomes continuous ven- trally with that of the other, so that in effect we have one large semicircular fold covering over the gill-slits. Event- ually the hinder free margin of the fold undergoes con- crescence with the body-wall, so that a single peribranchial cavity is formed about the gills. This cavity is closed all round except at one point, usually on the left side, but sometimes in the mid-ventral line, where it remains open as the poms brancldal
. Amphioxus and the ancestry of the vertebrates. Amphioxus; Sea squirts; Hemichordata. INTERNAL ANATOMY. 23 frog-tadpole the fold of one side becomes continuous ven- trally with that of the other, so that in effect we have one large semicircular fold covering over the gill-slits. Event- ually the hinder free margin of the fold undergoes con- crescence with the body-wall, so that a single peribranchial cavity is formed about the gills. This cavity is closed all round except at one point, usually on the left side, but sometimes in the mid-ventral line, where it remains open as the poms brancldalis, or so-called spiracidum. This comparison of Hux- ley's was extremely well taken, and although the two cavities, namely, the peri- branchial cavity of the frog- larva and the atrial chamber of Amphioxus, are probably by no means homologous, or genetically related to each other, still the close analogy that exists between them is most instructive, and yet, singular to say, it did not lead Huxley to a correct interpretation of the atrial chamber.^ Its true nature was at length established by Rolph in 1876. By comparing his own observations on the adult with those of Kowalevsky on the larva, Rolph came to the conclusion that the atrial cavity of Amphioxus originated by the growth of two folds of the body-wall over the gill- slits on each side, and by their subsequent fusion in the mid-ventral line except at one point, which remained open. Fig. 8. —Tadpole of Frog {Rana cla- mata) from ventral side. (Original.) cl. Dextrally placed cloacal aperture. m. Mouth, sp. spiraculum; the dotted line indicates the extent of the opercular chamber. /. 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 Willey, Arthur, 1867-. New York, London, Macmillan
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