Seaside Studies in Natural History. Marine Animals of Massachusetts Bay. Radiates.

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.

[fig 69]

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.

[fig 70]

[fig 71]

[fig 72]

[fig 73]

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.

[fig 74]

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.

[fig 75]

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.

[fig 76]

[fig 77]

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.

[fig 78]

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]

[fig 79]

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.

[fig 80]

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.

[fig 81]

[fig 82]

[fig 83]

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.

[fig 84]

[fig 85]

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.).

[fig 86]

[fig 87]

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.

[fig 88]

[fig 89]

[fig 90]

Among the most common of our Tubularians is a small, mossy Hydroid (Fig. 88), covering the rocks between tides, in patches of several feet in diameter. Fig. 89 represents a single head from this little mossy tuft greatly magnified, in which is seen the medusa bud arising from the stem by the process already described in the other Hydroids. In Fig. 90 we have the little Jelly-fish in its adult condition, about the size of a small walnut, with a wide circular opening, through which passes the long proboscis, hanging from the under surface of the disk to a considerable distance below its margin. The four tentacles are of an immense length when compared to the size of the animal. As a general thing, the tentacles are less numerous in the Tubularian Medusæ than in those arising from other Hydroids; they want also the singular limestone concretions found at the base of the tentacles in the Campanularian Medusæ. In Fig. 91 we have one of the Tubularian Medusæ (Turris vesicaria A. Ag.) which lifts a rather larger number of tentacles than is usual among these Jelly-fishes. We never find the tentacles multiplying almost indefinitely in them, as in Zygodactyla and Eucope. The little Jelly-fish described above is known as Sarsia, while its Hydroid is called Coryne. These names having been given to the separate phases of its existence before their connection was understood, and when they were supposed to represent two distinct animals. They are especially interesting with reference to the history of Hydroids in general, because they were among the first of these animals in whom the true relation between the different phases of their existence was discovered. Lesson named the Sarsia after the great Norwegian naturalist, Sars, to whom we owe so large a part of what is at present known respecting this curious subject of alternate generations.

[fig 91]

The Bougainvillia (Fig. 92), is one of our most common Jelly-fishes, frequenting our wharves as well as our sea-shore during the spring. The tentacles are arranged in four bunches or clusters at the junction of the radiating tubes with the circular tube, from which they may be seen extending in every direction whenever these animals remain quietly suspended in the water,—a favorite attitude with them, and one which they retain sometimes for days, seeming to make no effort beyond that of gently playing their tentacles to and fro (Fig. 92). These tentacles are capable of immense extension, sometimes to ten or fifteen times the diameter of the bell. The proboscis is not simple as in the Sarsia, but looks like a yellow urn suspended at its four corners from the chymiferous tubes. The oral opening is entirely concealed by clusters of shorter tentacles surrounding the mouth in a close wreath, on which the eggs are supported. A highly magnified branch of the Hydroid stock from which this Medusa arises is represented in Fig. 93. There we see the little Jelly-fishes in different degrees of development on the stem, while in Figs. 94-97 they are given separately and still more enlarged. In Fig. 94 the outline of the Jelly-fish is still oval, the proboscis is but just formed, and the tentacles appear only as round swellings or knobs. In Fig. 95 a depression has taken place at the upper end, presently to be an opening, the proboscis is enlarged, and the tentacles lengthened, but still turned inward. In Fig. 96 the appendages of the proboscis are quite conspicuous, the tentacles are turned outward, and the Jelly-fish is almost ready to break from its attachment, having assumed its ultimate outline. Fig. 97 represents it just after it has separated from the stem, when it has only two tentacles at each cluster and simple knobs around the mouth, instead of the complicated branching tentacles of the adult.

[fig 92]

[fig 93]

[fig 94]

[fig 95]

[fig 96]

[fig 97]

There are several other Tubularians common in our waters which should not be passed over without mention, although as this little book is by no means intended as a complete text-book, but rather as a volume of hints for amateur collectors, we would avoid as much as possible encumbering it with many names, or with descriptions already given in more comprehensive works. This Tubularia is interesting, however, from the fact that the Medusæ buds are never freed from the stem, and do not develop into full-grown Jelly-fishes, but always remain abortive. Fig. 98 represents one head of such a Hydroid with the Medusæ buds pendent from it in a thick cluster, while in Fig. 99 we have a few of them sufficiently magnified to show that, though presenting the four chymiferous tubes, they are otherwise exceedingly simple in structure, as compared with the free Jelly-fishes.

[fig 98]

[fig 99]

Hydractinia. (Hydractinia polyclina Ag.)

This is another Tubularian, covering the surface of rocks in tide-pools, or attaching itself upon shells inhabited by hermit crabs. Indeed it was upon these shells that the Hydractinia was first noticed, and it was long supposed that the wanderings to which the little colony was thus subjected were necessary for its healthy development. But subsequent observations have shown that it attaches itself quite as frequently to the solid rock as to these nomadic shells. It has a rosy color, and, being very small, it looks, until one examines it closely, more like a thick red carpet of soft moss, than like a colony of animals. These communities are distinct in sex, the fertile individuals in each being either all male or all female. In Fig. 100 we have a portion of a female colony, representing one fertile head, in which the buds are crowded with Medusæ; one sterile head, surrounded by its wreath of tentacles; and still another member of the society whose office is not fully understood, unless it be that of a kind of purveyor, catching food for the rest. Fig. 101 represents the corresponding individuals taken from a male colony. The sex makes little difference in the appearance of the reproductive heads. All the individuals of a Hydractinia colony are connected at the base by a horny network, rising occasionally into points of a conical or cylindrical shape. This polymorphism among the Tubularians is another evidence of the relation between the Siphonophoræ, or floating Hydroids, and the fixed Hydroids.

[fig 100]

[fig 101]

Among our Medusæ derived from a Tubularian stock is the Hybocodon, viz. the hunchbacked Medusa (Fig. 102), a singular little Jelly-fish, odd and unsymmetrical in shape, as its name indicates, and interesting from its relations to one of our floating communities, the Nanomia, presently to be described. Instead of the evenly proportioned bell of the ordinary Medusæ, the Hybocodon has a one-sided outline (Fig. 102), one large tentacle only being fully developed, while the others remain always abortive, so that the whole weight of the structure is thrown on one half of the bell. Upon this large tentacle small Jelly-fishes, similar to the original, are produced by budding, this process going on till ten or twelve such Jelly-fishes (Fig. 103) may be seen suspended from the tentacle. Up to this time it has remained connected with the Hydroid from which it arises, a rather large Tubularian, usually growing singly (Fig. 104), and of a deep orange-red in color. But at this stage of its existence it frees itself, and leads an independent life hereafter, swimming about with a quick, darting motion. In the account of the Nanomia, the homology between its scale, or abortive Medusa, and the Hybocodon, is traced in detail, and I need only allude to it here. Though this Medusa is so peculiar in appearance, the Tubularian from which it is derived is very like the Tubularia Couthouyi, already described. This is one of the instances before alluded to, in which closely allied forms give rise to very dissimilar ones, or, as in many cases, the very reverse of this takes place, and closely allied forms arise from very dissimilar ones.

[fig 102]

[fig 103]

[fig 104]

Besides the budding at the base of the tentacle, as in Hybocodon, we find another mode of development among Hydroid Medusæ, viz. that of budding from the proboscis. One of our most common little Jelly-fishes, the Dysmorphosa (Fig. 105), to which we owe the occasional blue phosphorescence of the sea, so brilliant at times, buds in this manner. Fig. 105 represents an adult Dysmorphosa, on the proboscis of which may be seen three small buds in different stages of development. In Fig. 106 the proboscis is more enlarged, showing one of the little Jelly-fishes similar to the parent, just ready to drop off. We need not wonder at the immense number of these animals, with which the sea actually swarms at times, when we know that as fast as they are dropped, and it takes but a few days to complete their development, they each begin the same process; so that in the course of a week or ten days one such Medusa, supposing it to have produced six buds only, will have given rise to forty-two Jelly-fishes, thirty-six of which may be equally prolific in the same short period. These Medusæ budding thus, and swimming about, carrying their young with them, bear such a close resemblance to the floating communities of Hydroids formerly known as Siphonophoræ, that did we not know that some of them arise from Tubularians, it would be natural to associate them with the Siphonophoræ.

[fig 105]

[fig 106]

The Nanomia (Fig. 115), our free floating Hydroid, consists, when first formed, of a single Hydra containing an oblong oil bubble (Fig. 107). The whole organisation of such a Hydra is limited to a simple digestive cavity; it has, in fact, but one organ, and one function, and consists of an alimentary sac resembling the proboscis of a Medusa (Fig. 107); the oil bubble is separated from it by a transverse partition, and has no connection with the cavity. Presently, between the oil bubble and the cavity arise a number of buds of various character (Fig. 108), which we will describe one by one, beginning with those nearest the oil bubble, since these upper members of the little swimming community bear a very important part in its history. The infant community (Fig. 108) passes rapidly into the stage represented in Fig. 109, and then through all the stages intermediate between this and the adult, shown in its natural size in Fig. 115. The upper buds enlarge gradually, and soon take upon themselves a perfect Medusa structure (Fig. 110), with the exception of the proboscis, the absence of which is easily understood, when we find that these Medusæ, serve the purpose of locomotion only, having no share in the function of feeding the community, so that a digestive apparatus would be quite superfluous for them. In every other respect they are perfect Medusæ, attached to the Hydra as the Medusa buds always are when first formed, having the (four) chymiferous tubes, characteristic of all Hydroid Medusæ, radiating from the centre to the periphery; two of these tubes are very winding, as may be seen in Fig. 110, while the other pair are straight. The Medusæ themselves are heart-shaped in form, depressed at the centre of the upper surface, and bulging on either side into wing-like expansions, where they join the stem. These expansions interlock with one another, crossing nearly at right angles. The Medusæ-like buds are the swimming bells; by their contractions, alternately taking in and throwing out the water, they impel the whole community forward, so that it seems rather to move like one animal, than like a combination of individuals.

[fig 107]

[fig 108]

[fig 109]

[fig 110]