Part 30

PLATE 30.

P. H. GOSSE, del. LEIGHTON, BROS.

CYDIPPE. COMMON MEDUSA. LARVAL FORMS. ]

INFANT FORMS.

The most interesting circumstances in the history of this large jelly-fish are the wonderfully varied phases through which it passes in the earlier stages of its existence. Along the margins of the lengthened flaps of the polypite there are found remarkable pouches, within which the ova are placed, and whence they are hatched in the form of soft flat animalcules, capable of swimming by means of cilia. This has been called a planula. After swimming a while, it alters its form to that of a pear, and presently adheres by its slender end to a sea-weed or rock under water, hanging downward. A depression now appears in the larger end, which deepens and forms a mouth and stomach, and the little planula has assumed a polype-form. Four tiny warts now spring from the margin of the mouth, which lengthen into tentacles; four more then shoot in the interspaces; these eight increase to sixteen, then to thirty-two, all at the same time acquiring great length. In this stage, in which it is very common in our aquaria, it has been supposed a new animal, and has been named Hydra tuba. The space between the margin and the mouth has widened into an “umbrella,” and the mouth has protruded into a polypite. The whole is of a translucent white hue, and the body without the tentacles is ordinarily about one-sixth of an inch high.

This stage sometimes lasts for years without further change, except that creeping root-threads shoot from the attached base, which send up at intervals buds that grow into Hydræ; and buds break out from different parts of the body itself, which likewise develop themselves, the form in both cases being exactly similar to that of the present Hydra tuba. Thus we frequently find numerous colonies of these tiny creatures crowded together.

At length a change takes place. The body enlarges both in length and thickness, and begins to show traces of rings or segments, as if it had been tied tightly round with threads at regular intervals. In this stage it has been described under the name of Scyphistoma. These cuts deepen, and the segments thus marked off become hollow; and so they resemble a pile of tiny saucers set one within another, each of which is now divided at its rim into eight teeth. In this stage it has been once more named, as if an independent animal, Strobila.

All this time the tentacles have been set around the terminal margin, but now these are absorbed, and a new set rapidly spring from the basal segment. The saucers become very loosely attached; at length the end one breaks away and swims through the sea, as a true Medusa, though no more than a sixth of an inch wide, pumping as it goes in proper parental wise. Others quickly follow, and thus a colony of tiny swimming jelly-fishes are shooting hither and thither in the liveliest manner. Strange to say, these little Medusæ, which as to details differ much from their adult form, have been again described, under the name of Ephydra; all these appellations indicating the assumptions of various naturalists, who found the little creatures in their respective stages, without knowing their previous history, that each was an independent form of animal life.

CYDIPPE.

As the closer and more severe scrutiny of anatomical structure has induced modern zoologists to separate the Lucernaria from its formerly assigned alliance with the Sea-anemones, and to associate it with the Medusæ, it is interesting to remark that the scales of justice have been maintained in equipoise by the like shifting of a member from the Medusæ to the Anemones. The latter animal is one familiar to most haunters of the shore, and invariably admired as one of the most charming of the many lovely forms that throng the summer seas; it is the sweet little Beröe, or Cydippe. Indeed at first sight you would be little disposed to admit the propriety of the transfer in this case, for certainly the active glittering globule of pure crystal appears to possess much more resemblance to one of the smaller Medusæ, the Sarsia, for instance,--than to a daisy or a beadlet. But naturalists look beneath the surface: and they find that, with important peculiarities, the internal economy of the Cydippe, and specially its digestive apparatus, are modelled rather on the type of the latter than of the former.

We will not, however, trouble ourselves now with these elaborate matters, but rather look at the exterior and obvious characters of the charming little pet, which is disporting itself in this vase of sea-water on our table. It is a globe of pure colourless jelly, about as big as a small marble, often having a little wart-like swelling at one of its poles, where the mouth is placed. At the other end there are minute orifices; and between the two passes the stomach, of a form which is flat, or wider in one diameter than in the other.

If the stomach be considered as the axis of the globe, and the two extremities as its poles, the meridians of longitude are well represented by eight narrow bands, situated on the surface, which do not, however, reach either pole. Along the course of each of these meridional bands are fixed at close intervals minute square moveable plates, whose outer edges are set with strong cilia, like the teeth of a comb. These are the locomotive organs, and most effective they are. They are used like the paddles of a steamer, the little animal beating the water with them in rapid and regular succession, their minute subdivision causing the rays of light, especially when in the sun, to play along these bands, with the most brilliant prismatic colours; while their vigorous strokes cause the globe to shoot hither and thither through the water with remarkable power.

Within the clear substance of the Cydippe, on each side of the stomach, there is excavated a capacious cavity, which communicates by a canal with the surface, near the equator. Within each cavity is fixed a tentacle of great length and slenderness, which the animal can at pleasure shoot out of the orifice, and allow to trail through the water, shortening, lengthening, twisting, or coiling it at will; or, on the other hand, quickly contract it into a tiny ball, and withdraw it wholly within the proper cavity. A peculiarity, which imparts an inexpressible charm to this apparatus, is, that, throughout the length of this attenuate white thread, short threadlets are given off at regular intervals, which can be coiled or straightened, lengthened or shortened, individually. They proceed only from one side of the thread-like tentacle, though, from the slight twisting of the axis, they seem now to project on one side, now on another.

It has been well observed that of the grace and beauty which the entire apparatus presents in the living animal, or the marvellous ease and rapidity with which it can be alternately contracted, extended, and bent at an infinite variety of angles, no verbal description can sufficiently treat. Fortunately this little beauty is so common in summer and autumn on all our coasts, that few who use the surface-net can possibly miss its capture. So lovely a creature is worthy of a poet’s description: it has received it.

“Now o’er the stern the fine-mesh’d net-bag fling, And from the deep the little Beroë bring; Beneath the sunlit wave she swims conceal’d By her own brightness;--only now reveal’d To sage’s eye, that gazes with delight On things invisible to vulgar sight. When first extracted from her native brine, Behold a small round mass of gelatine, Or frozen dew-drop, void of life and limb: But round the crystal goblet let her swim ’Midst her own element; and lo! a sphere Banded from pole to pole; a diamond clear, Shaped as bard’s fancy shapes the small balloon To bear some sylph or fay beyond the moon. From all her bands see lucid fringes play, That glance and sparkle in the solar ray With iridescent hues. Now round and round She wheels and twirls; now mounts, then sinks profound. Now see her like the belted star of Jove, Spin on her axis smooth, as if she strove To win applause--a thing of conscious sense Quivering and thrilling with delight intense. Long silvery cords she treasures in her sides, By which, uncoil’d at times, she moors and rides; From these, as hook-hairs on a fisher’s line, See feathery fibrils hang in graceful twine, Graceful as tendrils of the mantling vine These swift as angler by the fishy lake Projects his fly the keen-eyed trout to take, She shoots with rapid jerk to seize her food, The small green creatures of crustaceous brood; Soon doom’d herself a ruthless foe to find, When in Actinia’s arms she lies entwined. Here, prison’d by the vase’s crystal bound, Impassable as Styx’s nine-fold round, Quick she projects, as quick retracts again, Her flexile toils, and tries her arts in vain; Till languid grown, her fine machinery worn By rapid friction, and her fringes torn, Her full round orb wanes lank, and swift decay Pervades her frame, till all dissolves away. So wanes the dew conglobed on rose’s bud; So melts the ice-drop in the tepid flood: Thus, too, shall many a shining orb on high That studs the broad pavilion of the sky,-- Suns and their systems fade, dissolve, and die.”

FOOTNOTES:

Physalia pelagica, of which a representation is given in the centre of Plate XXVIII. Some naturalists make two or three other species, but I do not think that their distinctive characters can as yet be depended on.

Gatherings of a Naturalist, p. 7.

I ought to say that, as usual in these stranded examples, the tentacles and suckers were so mutilated by washing on the shore, that I have been compelled to aid my observation by the figures of Eschscholtz and Huxley, on whose correctness I could depend.

See his Gatherings in Australasia, for much interesting information on both these animals.

Op. cit.

Sarsia tubulosa; a group is represented of the size of life, in the lower right-hand corner of Plate XXVIII.

Æquorea Forbesiana; this fine species forms the subject of Plate XXIX.

Aurelia aurita, represented (about one-fourth of the natural size) in Plate XXX. On the extreme right and left are seen the young, in the stages of Hydra, Strobila, and Ephydra.

Cydippe pileus, seen, of the size of life, near the upper left-hand corner of Plate XXX.

Dr. Drummond, in Trans. Roy. Irish Acad. for 1839.

XI.

NOVEMBER