attention is the ample gelatinous disc, the mass of "blubber,” as it appears when cast upon the sands-the contractile bell, or swimming-organ, of more or less transparency and most graceful proportions, by whose rhythmical movement the living Medusa is propelled through the water. This is both the locomotive organ and the float on which are suspended the various organs of prehension and digestion. It varies in form, though scarcely in grace, but is commonly a glassy hemisphere with lobed or sinuated margin, from which in many cases depend a multitude of fringe-like filaments, or a smaller number of long extensile arms (vide Plate LXX. fig. 10). These tentacular appendages are sensitive feelers, cast about in all directions in quest of prey, and also fishing-lines by means of which it is arrested and dragged towards the mouth. In some species they are present in enormous numbers, and are capable of extrordinary elongation. The gigantic Cyanca arctica, which sometimes attains a diameter of 7 feet, has these organs disposed in eight bunches round the margin of its disc-tangled masses of interlacing threads, in constant motion, which can be extended to a length of more than 120 feet! When it is remembered that this formidable offensive apparatus is endowed with the power of stinging violently, that these extensile filaments, which can be shot out to such amazing distances, are poisonous and paralyse as well as grasp, we may form some idea of the terrors that wait upon this floating mass of jelly.

Round the margin of the disc are ranged certain organs of vision, which may be regarded as the equivalent of the eye in more highly organised beings (Plate LXX. fig. 9a). Each of them consists of a spherical cluster of lenses, borne on a peduncle, and usually more or less protected by a hood-like covering. Professor H. J. Clark, who has carefully studied the intimate structure of these bodies-who has actually taken out the minute lenses and turned them about, so as to trace the curvature of the face-is of opinion that we have in them "all the elements of an optical apparatus sufficient to produce a distinct image." At any rate, there can be little doubt that they are lightperceiving organs, and serve in some way or other to direct the creature in its course. Agassiz remarks that "there can be no doubt that these animals perceive what is going on about them, and that they are very sensitive to changes in the condition of the atmosphere..... Even accidental disturbances are perceived by them, for when approached, however carefully, the change of their course, or the unusual rapidity with which they sink, show plainly that they are making the utmost efforts to escape. When approached with a dip-net it is evident, from the

* A. Agassiz, in his "Catalogue of North-American Medusa."

acceleration of their movements, that they are attempting to escape."

From the centre of the lower or concave surface of the discthe top of the domed cavity-is suspended the digestive sac, a somewhat four-sided, proboscis-like body, terminating in a quadrangular mouth, which we readily recognise as a polypite, in spite of its disguise in the adaptive dress that fits it for a locomotive existence. The angles of this mouth are extended into lobate processes, often of very considerable length (Plate LXX. fig. 10), which are hung with fringes and furbelows, and form a striking feature of the organism, projecting as they often do far beyond the opening of the swimming-bell. These appendages, which have been styled the "grasping arms," assist in securing the prey and conveying it to the mouth. They also in some cases subserve another and very different purpose; within their ample folds, marsupial pouches are, as it were, extemporised, in which the ova pass through certain stages of their development, and ripen into the perfect embryo. At its upper extremity the pendent digestive sac opens into a somewhat extensive cavity, from which a certain number of tubular prolongations are given off, which penetrate the substance of the disc, and, after dividing, and in some cases sub-dividing, and anastomosing, so as to form a complicated vascular network, terminate in a circular canal that runs round the disc a little within the margin. Through these tubular offshoots of the stomach, radiating through the inferior stratum of the swimming-bell, its contents are at once distributed, and applied to the nutrition of the whole structure. They compare with the central channel, traversing all the ramifications of the common flesh in the plant-like zoophyte, and communicating directly with the stomachs of its multitudinous hydræ, by which the prepared pabulum is conveyed throughout the length and breadth of its complex organism.

In close connection with the central cavity, just described as surmounting the digestive sac, and also with the radiating tubes concerned in the circulation, are found certain pouches within which the ovary and spermary are lodged. These pouches open into the body-cavity, and the ova pass into it in due time, and find their way through the mouth to the marsupia prepared for their reception during the further stages of their development. The deeply-coloured reproductive organs show distinctly through the transparent disc, and give rise to the cruciform figure which adorns the summit. So much may suffice in the way of structural detail. We have now the grand features of this organic type clearly before us.* It is less easy to describe the various

⚫ I have not referred to the partially developed membranous veil which in some species surrounds the margin of the disc, nor to the curious internal

beauty of form; to give an idea of the exquisite motion of the bell, pulsating with rhythmical regularity, and, by its alternate contraction and expansion, driving itself gently through the water; or of its equally exquisite rest, as it floats, balloon-like, upon the tranquil surface; to trace the curves of the flexile. tentacles, or to paint the delicate but vivid hues with which the glass-like fabric is tinted. Let us hear M. Lesson, who, whatever may be his merits as a classifier, manifests the true enthusiasm and sensibility of a naturalist in writing of his favourite tribe:-"Il est peu d'animaux plus variés et plus intéressants à connaître que les acalèphes. . . . Ils rivalisent avec les fleurs par l'éclat de leur coloration. Souvent les gemmes ne scintillent point avec plus d'éclat que certains d'entre eux. Vaguant solitaires ou par essaims de myriades d'individus sur la surface des mers par le temps de calme, cachés lors des orages ou lorsque les vagues se heurtent; et, cherchant un refuge dans les couches d'eau plus paisibles, ils viennent pendant la sérénité des nuits émailler le bleu azuré de la mer par une phosphorescence vive et merveilleuse.'*

The distinguished naturalist, who, under the pseudonym of Alfred Frédol, has given us so charming an account of "Le Monde de la Mer," thus refers to the coloration of the Medusa: "Sometimes the animal is colourless and of a transparency almost equal to that of crystal; sometimes it is slightly opaline, of a delicate blue, or a pale rose colour. In some cases it presents the most vivid tints and the most brilliant iridescence. In certain species, the central portions only are tinted red or yellow, blue or violet; the rest of the body is without colour." These are suggestions of a beauty which really cannot be described; for, after all, it is colour suffusing the most exquisitely delicate tissues, and in association with perfect grace of form and motion that constitutes the charm of these ocean-wanderers. A word on the luminosity of the Medusa. The Discophora are by no means principal agents in producing the phenomena of phosphorescence. Certain kinds only are luminous in any high degree, and though the large and brilliant ball is a striking feature in the general illumination, it must yield to the myriads of Noctiluca and other minute beings which enamel the surface of the ocean "like little constellations fallen from the skies," or cover the dark waves with soft yellow light, and

tentacles (?) which are grouped about the generative pouches, and constitute a distinctive characteristic of the Discophora, because my object is to fix attention upon the obvious peculiarities of the tribe, rather than upon structural minutiæ, however important.

"Histoire naturelle des Zoophytes. Acalèphes, par René-Primevère Lesson."

change the very foam into "sparkles of sea-fire."" The Medusa represented in our plate (Pelagia cyanella) is eminently phosphorescent. We owe some of the best observations we possess on the subject to Spallanzani, who had the opportunity of studying a luminous species of Discophore on the coast of Sicily, and has given us in his "Travels" an interesting account of the results which he obtained. He found that the light was intermittent, sometimes continuing for a quarter of an hour, half an hour, or more, and then being suddenly extinguished, and not reappearing for a considerable interval. He was led to believe that the phosphorescence was manifested strongly only so long as the Medusa oscillated uninterruptedly, and faded when it passed into a state of rest. He also ascertained that the light was more vivid during the contraction than during the expansion of the disc, and that the principal seat of it was the edge of the swimming-bell and the large tentacles. This localisation of the phosphorescence has been noticed in many of the hydroid medusiform zooids. In one species the light kindles in the central proboscis alone, and resembles a little lamp suspended in a crystal globe. Amongst the fixed polypites the phosphorescence is fitful in its manifestations; they kindle their lights when irritated, and quench them when left to themselves. Of the mode in which the luminosity is produced, and its precise relation to the economy of the animal, we know little or nothing.

The development of the Discophore is the next point which claims our attention; and though the remarkable series of facts first brought to light by the independent researches of Sars and Dalyell has become so familiar as to have lost much of its marvellous hue, the real interest of it is still fresh as ever; it still reads like a romance to the uninitiated, while the naturalist has, perhaps, hardly fathomed its full significance. As I have mentioned, there is some variation in the course of development. I propose to follow the line which may be regarded as normal and characteristic of the tribe, and in doing so to take special note of the parallelism between the history of the Discophore and of the Hydroid, and also of the points of divergence.

The Medusa, then, as we have seen it, is fully equipped for the discharge of the reproductive function; and at the proper season the laden ovaries yield up their contents, and the embryos make their way from the generative chamber through the digestive cavity and its oral opening, and lodge themselves (how it is hard to say) amongst the ample folds of the tenta

* The phrase is Nathaniel Hawthorne's, who had the keenest eye for all the aspects of nature, and who thus describes the phosphorescence, which he had witnessed from the deck of a steamer off the American coast.

cular fringes. There they complete their development, and finally leave the parental shelter, as ciliated, free-swimming planulæ. (Plate LXX. fig. 1, the ovum; fig. 2, the embryo.) In this condition they bear an exact resemblance to the corresponding term of the Hydroid life-series. They appear as cylindrical bodies, thickly clothed with vibratile cilia, and furnished with a mouth at one extremity. After a term of free existence the planule selects a site for permanent settlement, and, having made a suitable choice, fixes itself by the pole of the body opposite to that which bears the mouth, and exchanges its roving habit for a purely vegetative life (Plate LXX. fig. 3). Its shape and proportions have altered; it is now narrowed below, and expands above; the ciliary appendages are still retained for a time, but they have lost their activity, while around the extreme basal portion of the body, a very delicate film of chitine is in some cases soon developed (Plate LXX. fig. 3a). The presence of this horny sheath, which, according to Agassiz, is only characteristic of some species, is a very interesting point, as another feature common to the Discophores and the plant-like Hydroids. As development proceeds the body lengthens, the cilia totally disappear, the upper portion becomes broad and somewhat cup-shaped, and from its margin bud a number of thread-like arms, while in the midst of them rises a quadrangular proboscis, bearing the mouth on its summit. The product of the Medusan egg now appears as a well-developed polypite, the equivalent of the primary zooid of the Hydroid colony (Plate LXX. fig. 4). Yet with all the similarity between them, there is an element of unlikeness too. The rather deep, cup-shaped disc surrounding the prominent proboscis of the Discophore suggests the swimming-bell of the Medusa, and we feel that if it were turned adrift, to float on the water, with the mouth downwards, it would bear no slight family resemblance to its parent.† In a word, the polypite of the Discophora has more of a Medusan look than that of the Hydroids.

To pursue the history, the polypite multiplies the number of its arms until they become a goodly company, corresponding with the voracious appetite which it is their office to satisfy; they are very extensile, and when much elongated are like threads of gossamer waving through the water. The mouth is

This process of selection, often conducted with much apparent fastidiousness, may be witnessed in the case of the embryos of fixed animals generally.

+ It must be remembered that some of the hydroid polypites have the tentacles connected by a web-like membrane, plainly indicating the way in which the structural elements are modified, so as to convert the fixed hydra into the free medusan or sexual zooid.