Part 6

Let us begin with its earlier condition. When it first escapes from the parent Hydroid stock, the Oceania is almost spherical in form. (See Fig. 65.) The disk is divided by four chymiferous tubes, running from the summit to the margin, where they meet the circular tube in which they all unite. At this time, it has but two well-developed tentacles, opposite each other on the margin of the disk, just at the base of two of the chymiferous tubes (Fig. 66), while two others are just discernible in a rudimentary state, forming slight projections at the base of the two other tubes. Fig. 66 gives a view of the animal from below, at this stage of its growth, while Fig. 65 shows it in profile. It will be seen by the latter how very spherical is the outline of the disk at this period, while the proboscis, in which are placed the mouth and digestive cavity, is quite long, and hangs down considerably below the lower surface of the disk. As the animal advances in age the disk loses its spherical outline, and becomes much flattened, as may be seen in Fig. 67. It may be well to introduce here some explanation of the law according to which the different sets of tentacles follow each other in successive cycles of growth, since it is a law of almost universal application in Jelly-fishes and Polyps; and, owing to the smaller number and simpler arrangement of the tentacles in Oceania, it may be more easily analyzed in them than in many others, where the number and complication of the different sets of tentacles make it very difficult to trace their relation to each other during their successive growth. We have seen that the Oceania begins life with only two tentacles. These form the first set, and are marked with the number 1 in the subjoined diagram, which gives the plan of all the different sets in their regular order. The second set, marked 2, consists also of two, which are developed at equal distances between the first two, i.e. at right angles with them. The third set, however, marked 3, consists of four, as do all the succeeding sets, and they are developed between the first and second. The fourth set comes in between the first and third; the fifth between the third and second; the sixth between the first and fourth; the seventh between the fifth and second; the eighth between the third and fourth; the ninth between the fifth and third. The ultimate number of tentacles in the Oceania is thirty-two, or sometimes thirty-six, and the cycles always in twos or multiples of two. But whatever be the number included in the successive sets of tentacles, and the unit for the first set ranges from two to forty-eight, the law in different kinds of Jelly-fishes is always the same, the youngest set always forming between the oldest preceding set. Thus the fourth set comes in between the first and third, and the fifth between the second and third, the intervals occupied now by the fourth set, being limited by the first set of tentacles on one side, and by the third set on the other side, while the intervals occupied by the fifth set are bounded by the second and third sets.

The little spheres represented between the tentacles on the margin of the disk, in Figs. 65-67, are eye-specks, and these continue to increase in number with age; in this the Oceania differs from the Eucope, in which it will be remembered there were but two eye-specks in each quarter-segment of the disk throughout life. Fig. 68 represents the adult Oceania in full size, when it averages from an inch and a half to two inches in diameter. It is slow and languid in its movements, coming to the surface only in the hottest hours of the summer days; at such times it basks in the sun, turning lazily about, and dragging its tentacles after it with seeming effort. Sometimes it remains for hours suspended in the water, not moving even its tentacles, and offering a striking contrast to its former great activity when young, and to the lively little Eucope, which darts through the water at full speed, hardly stopping to rest for a moment. If the Oceania be disturbed it flattens its disk, and folds itself up somewhat in the shape of a bale (see Fig. 69), remaining perfectly still, with the tentacles stretching in every direction. When the cause of alarm is removed, it gently expands again, resuming its natural outline and indolent attitudes. The number of these animals is amazing. At certain seasons, when the weather is favorable, the surface of the sea may be covered with them, for several miles, so thickly that their disks touch each other. Thus they remain packed together in a dense mass, allowing themselves to be gently drifted along by the tide till the sun loses its intensity, when they retire to deeper waters. Some points, not yet observed, are still wanting to complete the history of this Jelly-fish. By comparing such facts, however, as are already collected respecting it, with our fuller knowledge of the same process of growth in the Eucope, Tima, and Melicertum, we may form a tolerably correct idea of its development. It is hatched from a Campanularia.

_Clytia_. (_Clytia bicophora_ AG.)

In Figs. 70-73 we have the Acalephian and Hydroid stages of the Clytia (Fig. 73), another very pretty little Jelly-fish, closely allied to the Oceania. When first hatched, like the Oceania, it is very convex, almost thimble-shaped (see Fig. 70), but a little later the disk flattens and becomes more open, as in Fig. 71. In Fig. 72, we have a branch of the Hydroid, a Campanularia, greatly magnified, with the annulated reproductive calycle attached to it, and crowded with Jelly-fishes ready to make their escape as soon as the calycle bursts. The adult Clytia (Fig. 73) is somewhat smaller and more active than the Oceania, and is easily recognized by the black base of its tentacles, at their point of juncture with the margin of the disk. It is more commonly found at night, than in the day-time, being nocturnal in its habits.

_Zygodactyla_. (_Zygodactyla groenlandica_ AG.)

Little has been known, and still less published, of this remarkable genus of Jelly-fish (Figs. 74, 75) up to the present time. The name Zygodactyla, or Twinfinger, was given to it by Brandt, from drawings made by Mertens, who had some opportunity of studying it in his journey around the world. These drawings were published in the Transactions of the St. Petersburg Academy. In the year 1848 Professor Agassiz read a paper upon one of the species of this genus belonging to our coast, before the American Academy, in which he called it Rhacostoma, not being aware that it had already received a name, and gave some account of its extraordinary phosphorescent properties. The name Rhacostoma must of course yield to that of Zygodactyla, which has a prior claim.

The average size of this Jelly-fish when full grown is from seven to eight inches in diameter; sometimes it may measure even ten or eleven, but this is rather rare. The light-violet colored disk is exceedingly delicate and transparent, its edge being fringed with long fibrous tentacles, tinged with darker violet at their point of juncture with the disk, and hanging down a yard and more when fully extended, though they vary in length according to the size of the specimen, and, in consequence of their contractile power, may seem much shorter at some moments than at others. The radiating tubes in this Jelly-fish are exceedingly numerous, the whole inner surface of the disk being ribbed with them. (See Figs. 74 and 75.) The ovaries follow the length of the tubes, though they do not extend quite to their extremity, where they join the circular tube around the margin of the disk; nor do they start exactly at the point where the tubes diverge from the central cavity, but a little below it. (Fig. 74.) Each ovary consists of a long, brownish, flat bag, split along the middle, so closely folded together that it seems like a flat blade attached along the length of the tube. Perhaps a better comparison would be to a pea-pod greatly elongated, with the edges split along their line of juncture, and attached to a tube of the same length. The ovaries are not perfectly straight, but slightly waving, as may be seen in Fig. 74, and these undulations are stronger when the ovaries are crowded with eggs, as is the case at the time of spawning.

The large digestive cavity hangs from the centre of the under side of the disk (Fig. 75), terminating in the proboscis, which, in this kind of Jelly-fish, is short in proportion to the diameter of the disk, while the opening of the mouth is very large. (Fig. 74.) It is unfortunate that a variety of inappropriate names, likely to mislead rather than aid the unscientific observer, have been applied to different parts of the Jelly-fish. What we call here digestive cavity, proboscis, and mouth, are, in fact, parts of one organ. An exceedingly delicate, transparent, filmy membrane hangs from the under side of the disk; that membrane forms the outer wall of the digestive cavity, which it encloses; it narrows toward its lower margin, leaving open the circular aperture called the mouth; this narrowing of the membrane is produced by a number of folds in its lower part, while at its margin these folds spread out to form ruffles around the edge of the mouth, and these ruffles again extend into the long scalloped fringes hanging down below.

The motion of these Jelly-fishes is very slow and sluggish. Like all their kind, they move by the alternate dilatation and contraction of the disk, but in the Zygodactyla these undulations have a certain graceful indolence, very unlike the more rapid movements of many of the Medusæ. It often remains quite motionless for a long time, and then, if you try to excite it by disturbing the water in the tank, or by touching it, it heaves a slow, lazy sigh, with the whole body rising slightly as it does so, and then relapses into its former inactivity. Indeed, one cannot help being reminded, when watching the variety in the motions of the different kinds of Jelly-fishes, of the difference of temperament in human beings. There are the alert and active ones, ever on the watch, ready to seize the opportunity as it comes, but missing it sometimes from too great impatience; and the slow, steady people, with very regular movements, not so quick perhaps, but as successful in the long run; and the dreamy, indolent characters, of which the Zygodactyla is one, always floating languidly about, and rarely surprised into any sudden or abrupt expression. One would say, too, that they have their aristocratic circles; for there is a delicate, high-bred grace about some of them quite wanting in the coarser kinds. The lithe, flexible form of the greyhound is not in stronger contrast to the heavy, square build of the bull dog, than are some of the lighter, more frail species of Jelly-fish to the more solid and clumsy ones. Among these finer kinds we would place the Tima. (Fig. 76.)

_Tima_. (_Tima formosa_ AG.)

One's vocabulary is soon exhausted in describing the different degrees of consistency in the substance of Jelly-fishes. Delicate and transparent as is the Tima, it has yet a certain robustness and solidity beside the Oceania, described above. In fact, all are gelatinous, all are more or less transparent, and it is not easy to describe the various shades of solidity in jelly. Perhaps they may be more accurately represented by the impression made upon the touch than upon the sight. If, for instance, you place your hand upon a Zygodactyla, you feel that you have come in contact with a substance that has a positive consistency; but if you dip your finger into a bowl where a Tima is swimming, and touch its disk, you will feel no difference between it and the water in which it floats, and will not be aware that you have reached it till the animal shrinks away from the contact.

The adult Tima, represented in Fig. 76, is not more than an inch and a half or two inches in diameter. Instead of countless tubes diverging from the digestive cavity to the margin of the disk, as in the Zygodactyla, there are but four. The digestive cavity in the Tima is much smaller than in the Zygodactyla, and is placed at the end of the proboscis, which is long, and hangs down far below the disk. This removal of the digestive cavity to the extremity of the proboscis gives to the tubes arising from it a very different and much sharper curve than they have in the Zygodactyla. In the Tima they start from the end of the proboscis, as may be seen in the wood-cut (Fig. 76), and then turn abruptly off, when they arrive at the under surface of the disk, to reach its margin. The disk has, as usual, its veil and its fringe of tentacles; the tentacles in the full-grown Tima are few,--seven in all the four intermediate spaces between the tubes, with one at the base of each tube, making thirty-two in all. The ovaries, which are milk-white, follow the line of the tubes, as in the Zygodactyla, and have very undulating folds when full of eggs. The tubes meet in the digestive cavity, the margin of which spreads out to form four ruffled edges that hang down from it. One of these ruffles, considerably magnified, is represented in Fig. 77. In Fig. 78 we have a portion of the Hydroid stock from which this Jelly-fish arises, also greatly magnified. The Tima is very active, yet not abrupt in its motions; but when in good condition it is constantly moving about, rising to the surface by the regular pulsations of the disk, or swimming from side to side, or poising itself quietly in the water, giving now and then a gentle undulation to keep itself in position.

Though not a very frequent visitor of our shores, the appearance of the Tima is not limited by the seasons, since they are found at all times of the year. It is a fact, unexplained as yet, that the Tima and many other Jelly-fishes are never seen except when full grown. What may be the haunts and habits of these animals from the time of their hatching till they make their appearance again in the adult condition, is not known, though it is probable that they remain at the bottom during this period, and only come to the surface to spawn. This impression is confirmed by the observations made upon a very young Cyanea which was kept for a long time in confinement; but a question of this kind cannot of course be settled by a single experiment.[6]

[Footnote 6: Since the above was written, I have had an opportunity of learning some additional facts respecting the habits of the young Cyanea, which may, perhaps, apply to other Jelly-fishes also. Having occasion to visit the wharves at Provincetown at about four o'clock one morning, I was surprised to find thousands of the spring brood of Cyaneæ, hitherto supposed to pass the early period of their existence wholly in deep water, floating about near the surface. They varied in size, some being no larger than a three-cent-piece, while others were from an inch in diameter to three inches. It would seem that they make their appearance only during the earliest morning hours, for at seven o'clock, when I returned to the same spot, they had all vanished. It may be that other young Medusæ have the same habits of early rising, and that instead of coming to bask in the midday sunshine, like their elders, they prefer the cooler hours of the dawn. (_A. Agassiz_.)]

_Melicertum_. (_Melicertum campanula_ PÉR. et LES.)

A pretty Medusa, smaller and far more readily obtained than the Tima, is the Melicertum. (Fig. 80.) Its disk has a yellowish hue, and from its margin hangs a heavy row of yellow tentacles, while the eight ovaries (Fig. 79) are of a darker shade of the same color. This little golden-tinted Jelly-fish, moving through the water with short, quick throbs, produced by the rapid rise and fall of the disk, is a very graceful object. Its bright color, made particularly prominent by the darker undulating lines of the ovaries, which become very marked near the spawning season, renders it more conspicuous in the water than one would suppose from its size; for it does not measure more than an inch in height when full grown. (See Fig. 80.)

_Development of Melicertum and Tima_.

In the Melicertum and Tima we have had the good fortune to trace the process by which the eggs are changed into Hydroid communities. If any one has a curiosity to follow for themselves this singular history of alternate generations, the Melicertum is a good subject for the experiment, as it thrives well in confinement. After keeping a number of them in a large glass jar for a couple of days at the time of spawning, it will be found that the ovaries, which were at first quite full of eggs, are emptied, and that a number of planulæ; are swimming about near the bottom of the vessel. After a day or two the outline of these planulæ, spherical at first, becomes pear-shaped (see Fig. 81), and presently they attach themselves by the blunt end to the bottom of the jar. (Fig. 82.) Thus their Hydroid life begins; they elongate gradually, the horny sheath is formed around them, tentacles arise on the upper end, short and stunted at first, but tapering rapidly out into fine flexible feelers, the stem branches, and we have a little Hydroid community (Fig. 83), upon which, in the course of the following spring, the reproductive calycles containing the Medusæ buds will be developed, as in the case of the Eucope and Clytia. The Tima passes through exactly the same process, though the shape of the planulæ and the appearance of the young differ from that of the Melicertum, as may be seen in Fig. 78, where a single head of the Tima Hydroid, greatly magnified, is represented. By combining the above observations upon the development of the Hydroids of the Melicertum and Tima with those previously mentioned upon the young Medusa arising from reproductive calycles in the Eucope and Clytia, we get a complete picture of all the changes through which any one of these Hydroid Medusæ passes, from its Hydroid condition to the moment when it enters upon an independent existence as a free Jelly-fish.

(_Laomedea amphora_ AG.)

The Medusæ of the Campanularians are not all free. On the contrary, in many of the species they always remain attached to the Hydroid, never attaining so high a development as the free Medusæ, and withering on the stem after having laid their eggs. Such is the _Laomedea amphora_, quite common on all the bridges connecting Boston with the country, where, on account of the large amount of food brought down from the sewers by the river, they thrive wonderfully, growing to a great size, sometimes measuring from a foot to eighteen inches in height.

_Sertularians_.

The Sertularians form another group of Hydroids closely allied to the Campanularians, though differing from them in the arrangement of the sterile Hydræ upon the stem. Among these one of the most numerous is the Dynamena (_Dynamena pumila_ Lamx., Fig. 84), which hangs its yellowish fringes from almost every sea-weed above low-water-mark. It is especially thick and luxuriant on the fronds of our common _Fucus vesiculosus_. The color is usually of a pale yellow, though sometimes it is nearly white, and when first taken from the water it has a glittering look, such as a white frost leaves on a spray of grass. Fig. 84 represents such a cluster in natural size, while Fig. 85 shows a piece of the stem highly magnified, with a reproductive calycle attached to the side of a sterile Hydra stem. Many of these Sertularian Hydroids assume the most graceful forms, hanging like long pendent streamers from the Laminaria, or in other instances resembling miniature trees. One of these tree-like Sertularians (_Dyphasia rosacea_ Ag.), abundant on all rocks in sheltered places immediately below low-water-mark, is represented in Fig. 86. In both these Sertularians the Medusæ wither on the stock, never becoming free. The free Medusæ of the Sertularians are only known in their adult condition in a single genus, which is closely allied to Melicertum, and which is produced from a Hydroid genus called Lafoea. Fig. 87 represents one of these young Sertularian Medusæ (_Lafoea cornuta_ Lamx.).

_Tubularians_.

In the Sertularian and Campanularian Hydroids we have found that the communities consist generally of a large number of small individuals, so small, indeed, that it is hardly possible at first glance to distinguish the separate members of these miniature societies. Among the Tubularians, on the contrary, the communities are usually composed of a small number of comparatively large individuals; and indeed these Hydroids may even grow singly, as in the case of the Hybocodon (Fig. 104), which attains several inches in height. There is also another general feature in which the Tubularians differ from both the other groups of Hydroids. In the latter, the horny sheath which encloses the stem extends to form a protecting calycle around the Hydra heads. This protecting calycle is wanting round the heads of the Tubularians, though their stems are surrounded by a sheath.

_Sarsia_. (_Coryne mirabilis_ AG.)