Since my communication of the above-cited Paper to the Royal Society on the 16th December, 1875, two important contributions to the Anatomy of
Antedon
have appeared—one by Dr. Ludwig, chiefly based on his study of
Antedon Eschrichtii
(“Zur Anatomie der Crinoiden,” Zeitschrift für Wissenschaftliche Zoologie, Bd. xxvi. 1876, p. 361, continued in Nachrichten von der Königl. Gesellschaft der Wissenschaf ten und der Gr. A.Universität-zu Göttingen, No. 5, Feb. 23, 1876), and the other by Prof. Greef, of Marburg (Sitzungsberichte der G-esellschaft zur Beförderung der gesammten Naturwissenschaften zu Marburg, January 1876), both of which seem to have been prompted by the appearance of Professor Semper’s short paper on the subject. These able observers fully concur with me, as to all essential particulars, in the account I have given of the triple canal-system of the arms, which M. Edmund Perrier not only could not himself find, but ventured to predict that no one else would find; in fact, Professor G-reef’s figure of a transverse section of an arm might have been copied from one of the drawings I have had by me for more than ten years, save for one slight additional feature. The German investigators also accept the correctness of the statements made by me in my First Memoir, that the “nerve” of Müller is really the genital rachis, and that Müller’s “vessel” in the arms is solid, not tubular, though neither is disposed to believe with me that this “axial cord” is a nerve. The character of a nerve, on the other hand, is assigned by Ludwig to a fibrillar band lying beneath the epithelial floor of the ventral furrow of the arms; which band had been independently noticed by my son, Mr. P. H. Carpenter (who is at present working in the laboratory of Professor Semper at Würzburg), in two of Professor Semper’s Philip-pine species,
Actinometra armata
and
A. nigra
, as also in
Antedon Eschrichtii
, in which it had been previously discovered by Ludwig. It is not nearly so distinct, however, in
A. rosaceus
; but its existence in that species was also independently recognized by Professor Huxley, who, like Ludwig, was led by his general view of the homologies of the Crinoids to regard it as a nerve. My son regards both the ventral band of Ludwig and my “axial cord” as belonging to the nervous system, being led to that conclusion, as regards the former, by its homology with the radial nerves of other Echinoderms, and, as regards the latter, by the very definite branching he has discovered in the axial cord of the arms of
Actinometra armata
and
A. nigra
—two pairs of branches running on each side towards the dorsal surface, and two towards the ventral, where he has distinctly traced their ramifications as far as the leaflets bounding the ventral furrow. Prof. Greef, on the other hand, describes the whole epithelial floor of the ventral furrow as a nerve, on the ground that its histological character resembles that of the nerves of other Echinoderms. Having recently had an opportunity of examining at Würzburg the very thin sections prepared by my son, I can say with certainty that the fibrillar band is quite distinct from the layer of columnar epithelium which it underlies; but it appeared to me to send off very minute fibrils that pass up between the cells of which that layer is composed.
A letter on a canal in the medulla spinalis of some quadrupeds. In a letter from Mr. William Sewell, to Everard Home, Esq. F. R. S
