Chapter III: Part 3

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.)

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.

_Bougainvillia_. (_Bougainvillia superciliaris_ AG.)

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.

_Tubularia_. (_Tubularia Couthouyi_ AG.)

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.

_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.

_Hybocodon_. (_Hybocodon prolifer_ AG.)

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.

_Dysmorphosa_. (_Dysmorphosa fulgurans_ A. AG.)

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.

_Nanomia_. (_Nanomia cara_ A. AG.)

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.

Besides these locomotive members, the community contains three kinds of Hydræ arising as buds from the primitive Hydra below the swimming bells, the latter remaining always nearest the oil bubble at the top, while the first Hydra, the founder of the community, in proportion as the new individuals are added, is gradually pushed downward, and remains always at the end of the string, the stem of which is formed by the elongated neck of the primitive Hydra. All the three sets of Hydræ have certain features in common, while they have other distinguishing characteristics marking them as distinct individuals. They are all accompanied by triangular shields (Fig. 111), arising with them at the same point on the parent stem, and all are furnished with tentacles hanging down from the summit of the Hydra at the side opposite the shield. These facts are important to remember, since we shall presently perceive, upon analyzing their parts, that these Hydræ have a close homology to the Hybocodon. The tentacles differ in structure as well as in number for each kind of Hydra. Having shown in what characters they agree, let us now take each set individually, and see what differences they present.

In the first set which we will examine the Hydra is open-mouthed. Like the original Hydra, it is only a digestive tube, similar in all respects to the proboscis of a Medusa-disk. Its only function is that of feeding, and it shows a laudable fidelity to its calling, being very constantly and earnestly engaged in the work. Let us add, however, that in this instance the occupation is not a wholly selfish one, since the cavity of every Hydra communicates with that of the stem, and the food taken in at these over-gaping mouths, is at once circulated through all parts of the community, with the exception of the oil bubble, from which it is excluded by the transverse partition dividing it from all the lower members of the stock. The shields share in this general nourishment of the compound body by means of chymiferous tubes extending toward the outer surface, and opening into the cavity of the stem. The mouth of this Hydra is very flexible (Fig. 111), expanding and contracting at the will of the animal, and sometimes acting as a sucker, fastening itself, leech-like, on the object from which it seeks to draw its sustenance. (See Fig. 111.) The tentacles attached to this set of Hydræ are exceedingly long and delicate. They arise in a cluster at the upper and inner edge of the Hydra, just at its point of juncture with the stem, and being extremely flexible and contractile, their long tendril-like sprays are thrown out in an endless variety of attitudes. (See Fig. 115.) Along the whole length of this kind of tentacle are attached little pendent knobs at even distances; Fig. 112 represents such a knob greatly magnified, and absolutely paved with lasso-cells, the inner and smaller ones being surrounded by a row of larger ones.

The second set of Hydræ (Fig. 113), are also open-mouthed, corresponding with those described above, in everything except the tentacles, which are both shorter and thicker, and are coiled in a corkscrew-like spiral. These are thickly studded for their whole length with lasso-cells. (See Fig. 113.)

In the third and last set of Hydræ (Fig. 114), the mouth is closed; they have, therefore, no share in feeding the community, but receive their nourishment from the cavity of the stem into which they open. They differ also from the others in having a single tentacle instead of a cluster, and on this tentacle the lasso-cells are scattered at uneven distances (Fig. 114). The special function of these closed Hydræ is yet to be explained; they have oil bubbles at their upper end (see Fig. 111, the top Hydra), and though we have never seen them drop off, it seems natural to suppose that they do separate from the parent stock, and found new communities similar to those from which they arise.

The intricate story of this singular compound existence does not end here. There is still another set of individuals whose share in maintaining the life of the community is by no means the least important. Little bunches of buds, of a different character from any described above, may be seen at certain distances along the lower part of the stem. These are the reproductive individuals. They are clusters of imperfect sexual Medusæ, resembling the rudimentary Medusæ of Tubularia (Fig. 99), which are never freed from the parent stem, but discharge their contents at the breeding season. Like many other compound Hydroids, the sexes are never combined, in one of these communities; they are always either male or female, and as those with female buds have not yet been observed, we can only judge by inference of their probable character. Front what is already known, however, of Hydroid communities of a like description, we suppose that the process of reproduction must be the same in these, and that the female stocks of Nanomia give birth to small Jelly-fishes, the eggs of which become oil bubbles, similar to that with which our little community began. (Fig. 108.)

By the time all these individuals have been added along the length of the stem, the stem itself has grown to be about three inches long (Fig. 115), though the tentacles hanging from the various members of the community give to the whole an appearance of much greater length. The motion of this little string of living beings is most graceful. The oil bubble (Fig. 116) at the upper end is their float; the swimming bells immediately below it (Fig. 110), by the convulsive contractions of which they move along, are their oars. The water is not taken in and expelled again by all the bells at once, but first from all the bells on one side, beginning at the lower one, and then from all those on the opposite side, beginning also at the lower one; this alternate action gives to their movements a swinging, swaying character, expressive of the utmost freedom and grace. Whether such a little community darts with a lightning-like speed through the water, or floats quietly up and down, for its movements are both rapid and gentle, it always sways in this way from side to side. Its beauty is increased by the spots of bright red scattered along the length of the stock at the base and tips of the Hydræ, as well as upon the tentacles. The movements and attitudes of the tentacles are most various. Sometimes they shoot them out in straight lines on either side, and then the aspect of the whole thing reminds one of a tiny chandelier in which the coral drops make the pendants, or they may be caught up in a succession of loops or floating in long streamers; indeed, there is no end to the fantastic forms they assume, ever astonishing you by some new combination of curves. The prevailing hue of the whole community is rosy, with the exception of the oil bubble or float, which looks a bright garnet color when seen in certain lights.

Let us now compare one of the Hydræ hanging from the stem (Fig. 113) with the Hybocodon (Fig. 102). The reader will remember the unsymmetrical bell of this singular Medusa, one half of its disk more largely developed than the other, with the proboscis hanging from the centre, and the cluster of tentacles from one side. Let us now split the bell so as to divide it in two halves with the proboscis hanging between them; next enlarge the side where there are no tentacles, and give it a triangular outline; then contract the opposite side so as to draw up the cluster of tentacles to meet the base of the proboscis, and what have we? The proboscis now corresponds to the Hydra of our Nanomia, with the cluster of tentacles attached to its upper edge (Fig. 113), while the enlarged half of the bell represents the shield. If this homology be correct it shows that the Nanomia is not, as some naturalists have supposed all the Siphonophores to be, a single animal, its different parts being a mere collection of organs endowed with special functions, as feeding, locomotion, reproduction, &c., but that it is indeed a community of distinct individuals corresponding exactly to the polymorphous Hydroids, whose stocks are attached, such as Hydractinia, and differing from them only in being free and floating.

The homologies of the Siphonophoræ or floating Hydroids, with many of the fixed Hydroids, is perhaps more striking when we compare the earlier stages of their growth. Suppose, for instance, that the planula of our Melicertum (See Fig. 81) should undergo its development without becoming attached to the ground,--what should we then have? A floating community (Fig. 83), including on the same stock like the Nanomia, both sterile and fertile Hydræ, from the latter of which Medusæ bells are developed. The little Hydractinia community (Fig. 100), in which we have no less than four distinct kinds of individuals, each performing a definite distinct function, affords a still better comparison.

_Physalia_. (_Physalia Arethusa_ TIL.)

Among the most beautiful of the Siphonophores, is the well-known Physalia or Portuguese man-of-war, represented in Fig. 117. The float above is a sort of crested sac or bladder, while the long streamers below consist of a number of individuals corresponding in their nature and functions to those composing a Hydroid community. Among them are the fertile and sterile Hydræ (Fig. 118), the feeders and Medusæ bells (Fig. 119). The Physalia properly belongs to tropical waters, but sometimes floats northward, in the warm current of the Gulf Stream, and is stranded on Cape Cod. When found so far from their home, however, they have usually lost much of their vividness of color; to judge of their beauty one should see them in the Gulf of Mexico, sailing along with their brilliant float fully expanded, their crest raised, and their long tentacles trailing after them.

_Velella_. (_Velella mutica_ BOSC.)

Another very beautiful floating Hydroid, occasionally caught in our waters, though its home is also far to the south, is the Velella (Fig. 120). It is bright blue in color, and in form not unlike a little flat boat with an upright sail. Its Medusa (Fig. 121) resembles so much that of some of our Tubularians, that it has actually been removed on this account from the old group of Siphonophoræ, and placed next the Tubularians; another evidence of the close affinity between the former and the Hydroids.

MODE OF CATCHING JELLY-FISHES.

Not the least attractive feature in the study of these animals, is the mode of catching them. We will suppose it to be a warm, still morning at Nahant, in the last week of August, with a breath of autumn in the haze that softens the outlines of the opposite shore, and makes the horizon line a little dim. It is about eleven o'clock, for few of the Jelly-fishes are early risers; they like the warm sun, and at an earlier hour they are not to be found very near the surface. The sea is white and glassy, with a slight swell but no ripple, and seems almost motionless as we put off in a dory from the beach near Saunders's Ledge. We are provided with two buckets, one for the larger Jelly-fishes, the Zygodactyla, Aurelia, &c., the other for the smaller fry, such as the various kinds of Ctenophoræ, the Tima, Melicertum, &c. Beside these, we have two nets and glass bowls, in which to take up the more fragile creatures that cannot bear rough handling. A bump or two on the stones before we are fairly launched, a shove of the oar to keep the boat well out from the rocks along which we skirt for a moment, and now we are off. We pull around the point to our left and turn toward the Ledge, filling our buckets as we go. Now we are crossing the shallows that make the channel between the inner and outer rocks of Saunders's Ledge. Look down,--how clear the water is and how lovely the sea-weeds, above which we are floating, dark brown and purple fronds of the Ulvæ, and the long blades of the Laminaria with mossy green tufts between. As we issue from this narrow passage we must be on the watch, for the tide is rising, and may come laden with treasures, as it sweeps through it. A sudden cry from the oarsman at the bow, not of rocks or breakers ahead, but of "A new Jelly-fish astern!" The quick eye of the naturalist of the party pronounces it unknown to zoölogists, un-described by any scientific pen. Now what excitement! "Out with the net!--we have passed him! he has gone down! no, there he is again! back us a bit." Here he is floating close by us; now he is within the circle of the net, but he is too delicate to be caught safely in that way, so, while one of us moves the net gently about, to keep him within the space enclosed by it, another slips the glass bowl under him, lifts it quickly, and there is a general exclamation of triumph and delight,--we have him. And now we look more closely; yes, decidedly he is a novelty as well as a beauty. (See Fig. 122, _Ptychogena lactea_ A. Ag.) Those white mossy tufts for ovaries are unlike anything we have found before (Fig. 123), and not represented in any published figures of Jelly-fishes. We float about here for a while, hoping to find more of the same kind, but no others make their appearance, and we keep on our way to East Point, where there is a capital fishing ground for Medusæ of all sorts. Here two currents meet, and the Jelly-fishes are stranded as it were along the line of juncture, able to move neither one way nor the other. At this spot the sea actually swarms with life; one cannot dip the net into the water without bringing up Pleurobrachia, Bolina, Idyia, Melicertum, &c., while the larger Zygodactyla and Aurelia float about the boat in numbers. These large Jelly-fishes produce a singular effect as one sees them at some depth beneath the water; the Aureliæ, especially, with their large white disks, look like pale phantoms wandering about far below the surface; but they constantly float upward, and if not too far out of reach, one may bring them up by stirring the water under them with the end of the oar.

When we have passed an hour or so floating about just beyond East Point, and have nearly filled our buckets with Jelly-fishes of all sizes and descriptions, we turn and row homeward. The buckets look very pretty as they stand in the bottom of the boat with the sunshine lighting up their living contents. The Idyia glitters and sparkles with ever-changing hues, the Pleurobrachiæ dart about, trailing their long graceful tentacles after them, the golden Melicerta are kept in constant motion by their quick, sudden contractions, and the delicate transparent Tima floats among them all, not the less beautiful because so colorless. There is an unfortunate Idyia, who, by some mistake, has got into the wrong bucket with the larger Jelly-fish, where a Zygodactyla has entangled it among his tentacles and is quietly breakfasting upon it.

During our row the tide has been rising, and as we near the channel of Saunders's Ledge, it is running through more strongly than before, and at the entrance of the shallows a pleasant surprise is prepared us; no less than half a dozen of our new friends (the Ptychogena as he has been baptized), come to look for their lost companion perhaps, await us there, and are presently added to our spoils. We reach the shore heavily laden with the fruits of our morning's excursion.

The most interesting part of the work for the naturalist is still to come. On our return to the Laboratory, the contents of the buckets are poured into separate glass bowls and jars; holding them up against the light, we can see which are our best and rarest specimens; these we dip out in glass cups and place by themselves. If any small specimens are swimming about at the bottom of the jar, and refuse to come within our reach, there is a very simple mode of catching them. Dip a glass tube into the water, keeping the upper end closed with your finger, and sink it till the lower end is just above the animal you want to entrap; then lift your finger, and as the air rushes out the water rushes in, bringing with it the little creature you are trying to catch. When the specimens are well assorted, the microscope is taken out, and the rest of the day is spent in studying the new Jelly-fishes, recording the results, making notes, drawings, &c.

Still more attractive than the rows by day are the night expeditions in search of Jelly-fishes. For this object we must choose a quiet night, for they will not come to the surface if the water is troubled. Nature has her culminating hours, and she brings us now and then a day or night on which she seems to have lavished all her treasures. It was on such a rare evening, at the close of the summer of 1862, that we rowed over the same course by Saunders's Ledge and East Point described above. The August moon was at her full, the sky was without a cloud, and we floated on a silver sea; pale streamers of the aurora quivered in the north, and notwithstanding the brilliancy of the moon, they too cast their faint reflection in the ocean. We rowed quietly along past the Ledge, past Castle Rock, the still surface of the water unbroken, except by the dip of the oars and the ripple of the boat, till we reached the line off East Point, where the Jelly-fishes are always most abundant, if they are to be found at all. Now dip the net into the water. What genie under the sea has wrought this wonderful change? Our dirty, torn old net is suddenly turned to a web of gold, and as we lift it from the water heavy rills of molten metal seem to flow down its sides and collect in a glowing mass at the bottom. The truth is, the Jelly-fishes, so sparkling and brilliant in the sunshine, have a still lovelier light of their own at night; they give out a greenish golden light as brilliant as that of the brightest glow-worm, and on a calm summer night, at the spawning season, when they come to the surface in swarms, if you do but dip your hand into the water it breaks into sparkling drops beneath your touch. There are no more beautiful phosphorescent animals in the sea than the Medusæ; it would seem that the expression, "rills of molten metal" could hardly apply to anything so impalpable as a Jelly-fish, but, although so delicate in structure, their gelatinous disks give them a weight and substance; and at night, when their transparency is not perceived, and their whole mass is aglow with phosphorescent light, they truly have an appearance of solidity which is most striking, when they are lifted out of the water and flow down the sides of the net.