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THE DISCOPHORES, OR LARGE MEDUSÆ.
BY THE REV. THOMAS HINCKS, B.A.
NEW things can be less attractive or suggestive of grace and T brilliancy than the great lumps of jelly which so often strew the beach at certain seasons of the year. Yet these are the remains, sorely mangled in the rough collision between sea and shore, of some of the loveliest of living things. A jellyfish stranded, soiled by its contact with the sand, mutilated by the rush of the angry waters, its crystalline lustre dimmed, its vivid colours faded, its rhythmical movement stilled, is an unsightly ruin. Swept in by the advancing tide, with no power of resistance, and left an inert mass upon the shore, it is the very type of utter helplessness. But the Medusa, floating on the surface of a calm, clear sea beneath a summer sky, or gently borne onward by the rhythmical pulsations of its swimmingbell, its long fringe of slender filaments dependent from the margin of its crystal dome, and trailing after it in graceful eurves, is a model of exquisite form, and suggests nothing but happy freedom and the very luxury of motion, as it rises and falls in the yielding water. To the other elements of beauty which characterise it, vivid and varied colouring must be added; the hyaline disc is often adorned with blue, yellow, rose, purple, and other tints, while the fringes are also richly dyed.
We are not surprised to find that these beautiful creatures engaged the attention of the ancients, and have always excited the wonder of naturalists, if they have not always received very intelligent treatment at their hands. The singularity of their forms, and the profusion in which they occur in all seas, were sure to arouse curiosity at least, while the striking peculiarities of their structure and mode of life have stimulated and baffled the research of the anatomist and physiologist. “ All seas," say MM. Péron and Lesueur, whose names are most honourably
VOL. X.-XO. XXXIX.
connected with the history of the tribe, “ produce various kinds of these singular animals; they live amidst the Arctic waters of Spitzbergen, Greenland, and Iceland; they swarm under the fires of the Equator, while the great Southern Ocean is rich in numerous species. All maritime peoples appear to have known them from the highest antiquity, Philippides, Eupolis, Aristophanes, and Diphilus, before Aristotle, have mentioned them; and from the time of Pliny to our own days (the beginning of the 19th century) more than one hundred and fifty writers of all the nations of Europe have occupied themselves with their history.”* The true interpretation, however, of Medusan structure and life has been reached since the French naturalists wrote.
Aristotle, singling out a character which is by no means universal, fixed upon the tribe the name of sea-nettles( Acalephoe), in conjunction with the Actiniæ; and it has clung to the Medusæ from his time to the present. Many of the species, no doubt, sting, and sting severely; but many more seem to be destitute of the power, or to possess it in a very slight degree. The secretion of a poisonous or paralysing fluid, it may be remarked in passing, has an important relation to the diet of the jelly-fish, enabling it to deal with creatures much higher in the scale of being, and much more strongly built than itself. Other popular names for the Medusæ are founded on the phosphorescent properties which many of them manifest, and which form so striking a feature of their history. To the Italians they are “ Candellieri di mare;" the Arabs call them “Kandil el bahr" (lucerna marina). The resemblance between the rhythmical pulsation of the disc and the respiratory movement has suggested the designation of Pulmo marinus, or “sea-lungs," while our own "jelly-fish” and its less refined equivalent “sea-blubber," if not so poetical, are in their way as expressive as any.t
But turning from names to the things themselves, the Discophora constitute an Order of one of the two great branches into which the Cælenterate sub-kingdom divides itself, the Hydrozoa. It ranges alongside the Hydroid Zoophytes, as a parallel group; and while the two divisions have their capital features in common, they are separated by a sufficiently broad line of structural difference. The Hydroid includes within the compass of its individuality two principal elements, a fixed alimentary zooid or hydra, and a reproductive zooid, which is frequently organised for free and independent existence, on the Medusan
* “ Histoire générale des Méduses." Introduction.
+ Other trivial names are “Gelée de mer," “ Chapeau marin” (Mediterranean), “ Boule de mer," “ Capello di mare," &c.
type, but also frequently maintains its connection with the primitive stock in a less highly specialised form. So the Discophore has its two equivalent elements; a fixed polypite, whose functions are merely nutritive and vegetative, and a sexual zooid, developed from the former by gemmation, in which the Medusan structure reaches its highest grade, and which always leads a free oceanic life, while discharging its reproductive functions. In both groups there are exceptional genera in which the course of the life-history is modified by the suppression of the fixed element, and the Medusa is developed directly from the ovum, and not as a bud on a hydraform stock. Such forms, however, exhibit in each case the closest affinity to those which are produced in a more normal manner, and should clearly take their place amongst them in the ranks of the same division. In the two groups, then, the general plan of the life-history is identical, and there is a striking similarity in many of the structural features. The differences will be noticed when I come to describe the organisation of the Discophore in detail; but one salient point of contrast may be mentioned at once. Amongst the Hydroid Zoophytes the fixed vegetative element predominates ; the sexual members of the colony, though sometimes free and locomotive, are in a large number of cases as permanently attached to the parent organism as the flower to the plant, and where medusiform zooids are present, they are comparatively small, inconspicuous, and of a lower structural grade. But amongst the Discophores, the locomotive medusan element is altogether in the ascendant and reaches its culminating point; the polypites are small and insignificant, and of much the same pattern; the more highly specialised structure has gained upon the merely vegetative; and vagrancy, instead of plant-like fixity, is the characteristic of the tribe.
Let me first define more precisely the limits of the Order which forms the subject of the present paper. The Discophora embrace the large jelly-fishes (Plate LXX. fig. 10), or Swimmingpolypites—to borrow the expressive German name—whether developed directly from an egg or as buds from a hydra-like stock; and also an aberrant group of fixed Medusæ, the Lucernariida, of which I shall have more to say hereafter. The jelly-fish is both the most characteristic and the most familiar form under which the Discophore presents itself to us; and I shall at once attempt to sketch the leading features of its structure in plain as distinguished from technical language, merely premising that in a large proportion of cases it is not to be regarded as in itself a perfect animal, but only as one term of a life-series, which cannot be rightly interpreted alone. The feature which is most prominent and at once arrests