physiologist, "comes with the abhorred shears and slits the thin-spun life," we should be all agreed that in the case of these animals the life is so very thin-spun that any suggestion of abhorrence is on the face of it absurd.[1] [1] The relation of consciousness to the elaboration of nerve-centres throughout the animal kingdom is more fully considered in my work on "Mental Evolution in Animals" (Kegan Paul, Trench & Co.: 1883). CHAPTER I. STRUCTURE OF THE MEDUSÆ. To give a full account of the morphology, development, and classification of the Medusæ would be both unnecessary for our present purposes and impracticable within the space which is allotted to the present work.[2] But, for the sake of clearness in what follows, I shall begin by briefly describing such features in the anatomy of the jelly-fish as will afterwards be found especially to concern us. [2] Those who may desire to read an excellent epitome of our most recent knowledge on these subjects, may refer to Professor E. Ray Lankester's article in the "Encyclopædia Britannica" on "Hydrozoa," together with Professor Haeckel's Report on the Medusæ of the _Challenger_ Expedition. [Illustration: Fig. 1.--Sarsia (natural size).] In size, the different species of Medusæ vary from that of a small pea to that of a large umbrella having streamers a hundred feet long. The general form of these animals varies in different species from that of a thimble (Fig. 1) to that of a bowl, a parasol, or a saucer (see figures in subsequent chapters). Or we may say that the form of the animals always resembles that of a mushroom, and that the resemblance extends to a tolerably close imitation by different species of the various forms which are characteristic of different species of mushrooms, from the thimble-like kinds to the saucer-like kinds. Moreover, this accidental resemblance to a mushroom is increased by the presence of a central organ, occupying the position of, and more or leas resembling in form, the stalk of a mushroom. This organ is called the "manubrium," on account of its looking like the "handle" of an umbrella, and the term "umbrella" is applied to the other portion of the animal. The manubrium, like the umbrella, varies much in size and shape in different species, as a glance at any figures of these animals will show. Both the manubrium and umbrella are almost entirely composed of a thick, transparent, and non-contractile jelly; but the whole surface of the manubrium and the whole _concave_ surface of the umbrella are overlayed by a thin layer or sheet of contractile tissue. This tissue constitutes the earliest appearance in the animal kingdom of true muscular fibres, and its thickness, which is pretty uniform, is nowhere greater than that of very thin paper.