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 Geryonidæ, 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 Medusæ is given by Drs. Hertwig in their more recently published work on the nervous system and sense-organs of the Medusæ, 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 ring-nerve; so that in Haeckel's researches on Geryonia, whereby both true ganglion-cells and true nerve-fibres were first demonstrated as occurring in the Medusæ, 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.