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Archiv. f. Naturgeschichte," 1851, b. i.; McCrady, "Descriptions of Oceania, etc.," "Proceedings of the Elliot Society of Natural History," vol. i., 1859; L. Agassiz, "Contributions to the Acaliph of North America," "Memoirs of the American Academy of Arts and Sciences," vol. iii., 1860, vol. iv., 1862; Leuckart, "Archiv. f. Naturgeschichte,” Jahrg. 38, b. ii., 1872; Hensen, "Studien über das Gehörorgan der Decapoden," "Zeitchr. í. wiss. Zool.," bd. xiii., 1863; Semper, "Reisebericht," Zeitschr. f. wiss. Zool.," bd. xiii. vol. xiv.; Claus, Bemerkungen über Clenophoren und Medusen,” 'Zeitschr. f. wiss. Zool.," bd. xiv., 1864; Allman, "Note on the Structure of Certain Hydroid Medusa," "Brit. Assoc. Rep.," 1867; Fritz Müller, Polypen und Quallen von S. Catherina," "Archiv. f. Naturgesch.," Jahrg. 25, bd. i., 1859; also "Ueber die Rand-bläschen der Hydroidquallen," " Archiv. f. Anatomie und Physiologie," 1852; Haeckel, "Beiträge zur Naturgesch. der Hydromedusen," 1865; Eimer, "Zoologische Untersuchungen," Würzburg, "Verhandlungen der Phys.-med. Gesellschaft," N.F. vi. bd., 1874.

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The most important of these memoirs for us to consider are the two last. I shall subsequently consider the work of Dr. Eimer, which up to this date was of a purely physiological character. Professor Haeckel, who made his microscopical observations chiefly upon the Geryonida, described the nervous elements as forming a continuous circle all round the margin of the umbrella, following the course of the radial or nutrient tubes throughout

their entire length, and proceeding also to the tentacles and marginal bodies. At the base of each tentacle there is a ganglionic swelling, and it is from these ganglionic swellings that the nerves just mentioned take their origin. The most conspicuous of these nerves are those that proceed to the radial canals and marginal bodies, while the least conspicuous are those that proceed to the tentacles. Cells, as a rule, can only be observed in the ganglionic swellings, where they appear as fusiform and distinctly nucleated bodies of great transparency and high refractive power. On the other hand, the nerves that emanate from the ganglia are composed of a delicate and transparent tissue, in which no cellular elements can be distinguished, but which is longitudinally striated in a manner very suggestive of fibrillation. Treatment with acetic acid, however, brings out distinct nuclei in the case of the nerves that are situated in the marginal vesicles, while in those that accompany the radial canals ganglion-cells are sometimes met with.

A brief sketch of the contents of these and other memoirs on the histology of the Medusa is given by Drs. Hertwig in their more recently published work on the nervous system and sense-organs of the Medusa, and these authors point to the important fact that before the appearance of Haeckel's memoir, Leuckart was the only observer who spoke for the fibrillar character of the so-called marginal ringnerve; so that in Haeckel's researches on Geryonia, whereby both true ganglion-cells and true nervefibres were first demonstrated as occurring in the

Medusa, we have a most important step in the histology of these animals. Haeckel's results in these respects have since been confirmed by Claus, Grundzüge der Zoologie," 1872; Allman, “A Monograph of the Gymnoblastic or Tubularian Hydroids," 1871; Harting, "Notices Zoologiques," Niedlandisches "Archiv. f. Zool.," bd. ii., Heft 3, 1873; F. E. Schulze, " Ueber den Bau von Syncorzne Sarsii"; O. and R. Hertwig, "Das Nervensystem und die Sinnesorgane der Medusen."

The last-named monograph is much the most important that has appeared upon the histology of the Medusæ. I shall, therefore, give a condensed epitome of the leading results which it has established.

There is so great a difference between the nervous system of the naked and of the covered eyed Medusa, that a simultaneous description of the nervous system in both groups is not by these authors considered practicable. Beginning, therefore, with the naked-eyed division, they describe the nervous system as here consisting of two parts, a central and a peripheral. The central part is localized in the margin of the swimming-bell, and there forms a "nerve-ring," which is divided by the insertion of the "veil"* into an upper and a lower nerve-ring. In many species the upper nerve-ring is spread out in the form of a flattish layer, which

*This is the name given to a small annular sheet of tissue which forms a kind of floor to the orifice of the swimming-bell, through the central opening of which floor the manubrium passes. The structure is shown in Fig. 1.

is somewhat thickened where it is in contact with the veil. In these species the nerve-ring is only indistinctly marked off from the surrounding tissues. But in other species the crowding together of the nerve-fibres at the insertion of the veil gives rise to a considerable concentration of nervous structures; while in others, again, this concentration proceeds to the extent of causing a well-defined swelling of nervous tissue against the epithelium of the veil and umbrella. In the Geryonidæ this swelling is still further strengthened by a peculiar modification of the other tissues in the neighbourhood, which had been previously described by Professor Haeckel. In all species the upper nerve-ring lies entirely in the ectoderm. Its principal mass is composed of nervefibres of wonderful tenuity, among which are to be found sparsely scattered ganglion-cells. The latter are for the most part bi-polar, more seldom multipolar. The fibres which emanate from them are very delicate, and, becoming mixed with others, do not admit of being further traced. Where the nervous tissue meets the enveloping epithelium it is connected with the latter from within, but differs widely from it; for the nerve-cells contain a longitudinally striated cylindrical or thread-like nucleus which carries on its peripheral end a delicate hair, while its central end is prolonged into a fine nervefibre. There are, besides these, two other kinds of cells which form a transition between the ganglion and the epithelium cells. The first kind are of a long and cylindrical form, the free ends of which reach as far as the upper surface of the epithelium

The second kind lie for the most part under the upper surface. They are of a large size, and present, coursing towards the upper surface, a long continuation, which at its free extremity supports a hair. In some cases this continuation is smaller, and stops short before reaching the outer surface. Drs. Hertwig observe that in these peculiar cells we have tissue elements which become more and more like the ordinary ganglion-cells of the nerve-ring the more that their long continuation towards the surface epithelium is shortened or lost, and these authors are thus led to conclude that the upper nerve-ring was originally constituted only by such prolongations of the epithelium-cells, and that afterwards these prolongations gradually disappeared, leaving only their remnants to develop into the ordinary ganglion-cells already described.

Beneath the upper nerve-ring lies the lower nerve-ring. It is inserted between the muscletissue of the veil and umbrella, in the midst of a broad strand wherein muscle-fibres are entirely absent. It here constitutes a thin though broad layer which, like the upper nerve-ring, belongs to the ectoderm. It also consists of the same elements as the upper nerve-ring, viz. of nerve-fibres and ganglion-cells. Yet there is so distinct a difference of character between the elements composing the two nerve-rings, that even in an isolated portion it is easy to tell from which ring the portion has been taken. That is to say, in the lower nervering there are numerous nerve-fibres of considerable thickness, which contrast in a striking manner with

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