32 ME. p. H. CARPENTER OX THE GENUS ACTINOMETRA. The position of the point at which the two folds of perisome bounding the sides of the original anilulaeral groove meet and unite, varies extremely. The fusion may, though rarely, take place on the disk ; sometimes it is at the base of the arms, and sometimes not till near their middle or terminal portions. In any case, however, the fusion, whenever it occurs, is so complete that all trace of the original ambulacral groove is entirely obliterated. (§ 23) The bearings of this fact upon the different views advanced by Greeff and Ludwig respecting the homologies of the ambulacral grooves of the Crinoids will be best discussed at a later period, when the changes undergone by the various structures underlying the grooves are described and illustrated. One point, however, must be noticed here on account of its importance with respect to the two views now entertained regarding the nervous system of Comatula. As long ago as 18G5 it was stated by Dr. Carpenter that the cord which traverses the length of the arms between the subtentacular and coeliac canals, " and which was regarded by Professor Miiller as a nerve, really belongs to the reproductive apparatus. But it will also be shown that a regular system of branching fibres proceeding from the solid cord (described by Professor Miller as a vessel) that traverses the axial canal of each calcareous segment of the rays and arms, is traceable on the extremities of the muscular bundles i and reasons will be given for regarding these fibres as probably having the function of nerves, though not exhibiting their characteristic structure" During his residence in the Philippine Islands, Professor Semper had also discovered that the arm-nerve of Miiller is really a part of the generative system ; and in a short paper published some time after his return he announced this fact, and suggested at the same time, " dass der bisher immer als Gefass aufgefasste Strang im innern des Kalkskelettes ein Nervcnstrang sei, und dann ware wool das im Kelch liegende sogenannte Herz als cin Gano-lion anzuschcn" These observations of Dr. Carpenter's and Professor Scmpcr's were unfortunately overlooked for many years, so that even as late as 1874 Miiller's erroneous statements witli regard to the nervous system of Comatula were repeated in the valuable text-book of Gegenbaur and in many smaller Works. At the commencement of 187G, however, two very different views respecting the nervous system were put forward nearly simultaneously by Green and by Dr. Carpenter. The former ' described the whole floor of the ambulacral grooves on the arms and disk of Ant. rosacea as constituting a radial nervous system, starting from an oral nervous ring in the peristome, and corresponding " Ueber den Bau der Echinodermen. III. Mittlieilung" Sitzungsb. der Gesell. z. Boforder. d. gesamm. Natunviss. zu Marburg, ISTLI, No. 1 1 p. 1 55. " Beitr. z. Anat. der Crinoideen" Nachrichten von dor Konigl. Ocsclls. der Wissen-. u. dor G. A. Tjniversitiit zu Gottingen, 187G, No. 5, pp. 107, 108. " "Researches on this Structure, Physiology, and Development of Antedon rosaceus. Parti" Philos. Trans, vol. clvi. p. 705. • " Kurt anatomy. Bcmerk. iiber Comau7rt" Arbeit, aus d. zool-zootom. Instzuwurzburg, Band i. (1874), p. 262. Grundriss der vergleich. Anat. p. 22i. " Ueber den Bau dor Crinoidea" Jlarburg. Sitzungsb. 17C, No. 1, Jan. 13, p. 21.