Chapter II: Part 2

The life of the Jelly-fishes, with the exception of the Millepores and the like, is short in comparison to that of other Radiates. While Polyps live for many years, and Star-fishes and Sea-urchins require ten or fifteen years to attain their full size, the short existence of the Acaleph, with all its changes, is accomplished in one year. The breeding season being in the autumn, the egg grows into a Hydroid during the winter; in the spring the Jelly-fish is freed from the Hydroid stock, or developed upon it as the case may be; it attains its full size in the fall, lays its eggs and dies, and the cycle is complete. The autumn storms make fearful havoc among them, swarms of them being killed by the fall rains, after which they may be found thrown up on the beaches in great numbers. When we consider the size of these Jelly-fishes, their rapidity of growth seems very remarkable. Our common Aurelia measures some twelve to eighteen inches in diameter when full grown, and yet in the winter it is a Hydra so small as almost to escape notice. Still more striking is the rapid increase of our Cyanea, that giant among Jelly-fishes, which, were it not for the soft, gelatinous consistency of its body, would be one of the most formidable among our marine animals.

Before entering upon the descriptions of the special kinds of Jelly-fishes, we would remind our readers that the radiate plan of structure is reproduced in this class of animals as distinctly as in the Polyps, though under a different aspect. Here also we find that there is a central digestive cavity from which all the radiating cavities, whether simple or ramified, diverge toward the periphery. It is true that the open chambers of the Polyps are here transformed into narrow tubes, by the thickening of the dividing partitions; or in other words, the open spaces of the Polyps correspond to tubes in the Acalephs, while the partitions in the Polyps correspond to the thick masses of the body dividing the tubes in the Acalephs. But the principle of radiation on which the whole branch of Radiates is constructed controls the organization of Acalephs no less than that of the other classes, so that a transverse section across any Polyp (Fig. 1), or across any Acaleph (Fig. 50), or across any Echinoderm (Fig. 140), shows their internal structure to be based upon a radiation of all parts from the centre to the periphery.

That there may be no vagueness as to the terms used hereafter, we would add one word respecting the nomenclature of this class, whose aliases might baffle the sagacity of a police detective. The names Acalephs, Medusæ, or the more common appellation of Jelly-fishes, cover the same ground, and are applied indiscriminately to the animals they represent. The name Jelly-fish is an inappropriate one, though the gelatinous consistency of these animals is accurately enough expressed by it; but they have no more structural relation to a fish than to a bird or an insect. They have, however, received this name before the structure of animals was understood, when all animals inhabiting the waters were indiscriminately called fishes, and it is now in such general use that it would be difficult to change it. The name Medusa is derived from their long tentacular appendages, sometimes wound up in a close coil, sometimes thrown out to a great distance, sometimes but half unfolded, and aptly enough compared to the snaky locks of Medusa. Their third and oldest appellation, that of Acalephs,--alluding to their stinging or nettling property, and given to them and like animals by Aristotle, in the first instance, but afterwards applied by Cuvier in a more limited sense to Jelly-fishes,--is the most generally accepted, and perhaps the most appropriate of all.

The subject of nomenclature is not altogether so dry and arid as it seems to many who do not fully understand the significance of scientific names. Not only do they often express with terse precision the character of the animal or plant they signify, but there is also no little sentiment concealed under these jaw-breaking appellations. As seafaring men call their vessels after friends or sweethearts, or commemorate in this way some impressive event, or some object of their reverence, so have naturalists, under their fabrication of appropriate names, veiled many a graceful allusion, either to the great leaders of our science, or to some more intimate personal affection. The _Linnæa borealis_ was well named after his famous master, by a disciple of the great Norwegian naturalist; _Goethea semperflorens_, the ever-blooming, is another tribute of the same kind, while the pretty, graceful little Lizzia, named by Forbes, is one instance among many of a more affectionate reference to nearer friends. The allusions of this kind are not always of so amiable a character, however,--witness the "Buffonia," a low, noxious weed, growing in marshy places, and named by Linnæus after Buffon, whom he bitterly hated. Indeed, there is a world of meaning hidden under our zoölogical and botanical nomenclature, known only to those who are intimately acquainted with the annals of scientific life in its social as well as its professional aspect.

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CTENOPHORÆ.

The Ctenophoræ differ from other Jelly-fishes in their mode of locomotion. All the Discophorous Medusæ, as well as Hydroids, move by a rhythmical rise and fall of the disk, contracting and expanding with alternations so regular, that it reminds one of the action of the lungs, and seems at first sight to be a kind of respiration in which water takes the place of air. The Greeks recognized this peculiar character in their name, for they called them Sea-lungs. Indeed, locomotion, respiration, and circulation are so intimately connected in all these lower animals, that whatever promotes one of these functions affects the other also, and though the immediate result of the contraction and expansion of the disk seems to be to impel them through the water, yet it is also connected with the introduction of water into the body, which there becomes assimilated with the food in the process of digestion, and is circulated throughout all its parts by means of ramifying tubes. In the Ctenophoræ there is no such regular expansion and contraction of the disk; they are at once distinguished from the Discophoræ by the presence of external locomotive appendages of a very peculiar character. They move by the rapid flapping of countless little oars or paddles, arranged in vertical rows along the surface of the disk, acting independently of each other; one row, or even one paddle, moving singly, or all of them together, at the will of the animal; thus enabling it to accelerate or slacken its movements, to dart through the water rapidly, or to diminish its speed by partly furling its little sails, or, spreading them slightly, to poise itself with a faint, quivering movement that reminds one of the pause of the humming-bird in the air,--something that is neither positive motion, nor actual rest.[3]

[Footnote 3: The flappers of one side are sometimes in full
activity, while those of the other side are perfectly quiet or
nearly so, thus producing rotatory movements in every direction.]

These locomotive appendages are intimately connected with the circulating tubes, as we shall see when we examine the structural details of these animals, so that in them also breathing and moving are in direct relation to each other. To those unaccustomed to the comparison of functions in animals, the use of the word breathing, as applied to the introduction of water into the body, may seem inappropriate, but it is by the absorption of aerated water that these lower animals receive that amount of oxygen into the system, as necessary to the maintenance of life in them, as a greater supply is to the higher animals. The name of Ctenophoræ or comb-bearers, is derived from these rows of tiny paddles which have been called combs by some naturalists, because they are set upon horizontal bands of muscles, see Fig. 29, reminding one of the base of a comb, while the fringes are compared to its teeth. These flappers add greatly to the beauty of these animals, for a variety of brilliant hues is produced along each row by the decomposition of the rays of light upon them when in motion. They give off all the prismatic colors, and as the combs are exceedingly small, so that at first sight one hardly distinguishes them from the disk itself, the exquisite play of color, rippling in regular lines over the surface of the animal, seems at first to have no external cause.

_Pleurobrachia_. (_Pleurobrachia rhododactyla Ag_.)

Among the most graceful and attractive of these animals are the Pleurobrachia (Fig. 29), and, though not first in order, we will give it the precedence in our description, because it will serve to illustrate some features of the other two groups. The body of the Pleurobrachia consists of a transparent sphere, varying, however, from the perfect sphere in being somewhat oblong, and also by a slight compression on two opposite sides (Figs. 27 and 28), so as to render its horizontal diameter longer in one direction than in the other (Fig. 30). This divergence from the globular form, so slight in Pleurobrachia as to be hardly perceptible to the casual observer, establishing two diameters of different lengths at right angles with each other, is equally true of the other genera. It is interesting and important, as showing the tendency in this highest group of Acalephs to assume a bilateral character. This bilaterality becomes still more marked in the highest class of Radiates, the Echinoderms. Such structural tendencies in the lower animals, hinting at laws to be more fully developed in the higher forms, are always significant, as showing the intimate relation between all parts of the plan of creation. This inequality of the diameters is connected with the disposition of parts in the whole structure, the locomotive fringes and the vertical tubes connected with them being arranged in sets of four on either side of a plane passing through the longer diameter, showing thus a tendency toward the establishment of a right and left side of the body, instead of the perfectly equal disposition of parts around a common centre, as in the lower Radiates.

The Pleurobrachia are so transparent, that, with some preparatory explanation of their structure, the most unscientific observer may trace the relation of parts in them. At one end of the sphere is the transverse split (Fig. 27), that serves them as a mouth; at the opposite pole is a small circumscribed area, in the centre of which is a dark eye-speck. The eight rows of locomotive fringes run from pole to pole, dividing the whole surface of the body like the ribs on a melon. (Figs. 27, 28.) Hanging from either side of the body, a little above the area in which the eye-speck is placed, are two most extraordinary appendages in the shape of long tentacles, possessing such wonderful power of extension and contraction that, while at one moment they may be knotted into a little compact mass no bigger than a pin's head, drawn up close against the side of the body, or hidden within it, the next instant they may be floating behind it in various positions to a distance of half a yard and more, putting out at the same time soft plumy fringes (Fig. 29) along one side, like the beard of a feather. One who has never seen these animals may well be pardoned for doubting even the most literal and matter-of-fact account of these singular tentacles. There is no variety of curve or spiral that does not seem to be represented in their evolutions. Sometimes they unfold gradually, creeping out softly and slowly from a state of contraction, or again the little ball, hardly perceptible against the side of the body, drops suddenly to the bottom of the tank in which the animal floating, and one thinks for a moment, so slight is the thread-like attachment, that it has actually fallen from the body; but watch a little longer, and all the filaments spread out along the side of the thread, it expands to its full length and breadth, and resumes all its graceful evolutions.

One word of the internal structure of these animals, to explain its relation to the external appendages. The mouth opens into a wide digestive cavity (Figs. 27, 28), enclosed between two vertical tubes. Toward the opposite end of the body these tubes terminate or unite in a single funnel-like canal, which is a reservoir as it were for the circulating fluid poured into it through an opening in the bottom of the digestive cavity. The food in the digestive cavity becomes liquefied by mingling with the water entering with it at the mouth, and, thus prepared, it passes into this canal, from which, as we shall presently see, all the circulating tubes ramifying throughout the body are fed. Two of these circulating tubes, or, as they are called from the nature of the liquid they contain, chymiferous tubes, are very large, starting horizontally and at right angles with the digestive cavity from the point of junction between the vertical tubes (Fig. 30) and the canal. Presently they give off two branches, those again ramifying in two directions as they approach the periphery, so that each one of the first main tubes has multiplied to four, before its ramifications reach the surface, thus making in all eight radiating tubes. So far, these eight tubes are horizontal, all diverging on the same level; but as they reach the periphery each one gives rise to a vertical tube, running along the surface of the body from pole to pole, just within the rows of locomotive fringes on the outer surface, and immediately connected with them (Figs. 27, 28). As in all the Ctenophoræ, these fringes keep up a constant play of color by their rapid vibrations. In Pleurobrachia the prevailing tint is a yellowish pink, though it varies to green, red, and purple, with the changing motions of the animal. We have seen that the vertical tubes between which the digestive cavity is enclosed, start like the cavity itself from that pole of the body where the mouth is placed, and that, as they approach the opposite pole, at a distance from the mouth of about two thirds the whole length of the body, they unite in the canal, which then extends to the other pole where the eye-speck is placed. As it is just at this point of juncture between the tubes and the canal that the two main horizontal tubes arise from which all the others branch on the same plane (Figs. 27, 28), it follows that they reach the periphery, not on a level with the pole opposite the mouth, but removed from it by about one third the height of the body. In consequence of this the eight vertical tubes arising from the horizontal ones, in order to run the entire length of the body from pole to pole, extend in opposite directions, sending a branch to each pole, though the branch running toward the mouth is of course the longer of the two. The tentacles have their roots in two sacs within the body, placed at right angles with the split of the mouth. (Figs. 27, 30.) They open at the surface on the opposite side from the mouth, though not immediately within the area at which the eye-speck is placed, but somewhat above it, and at a little distance on either side of it. The tentacles may be drawn completely within these sacs, or be extended outside, as we have seen, to a greater or less degree, and in every variety of curve or spiral.

_Bolina_. (_Bolina alata_ AG.)

The Bolina (Fig 32), like the Pleurobrachia, is slightly oval in form, with a longitudinal split at one end of the body, forming a mouth which opens into a capacious sac or digestive cavity. But it differs from the Pleurobrachia in having the oral end of the body split into two larger lobes (Fig. 31), hanging down from the mouth. These lobes may gape widely, or they may close completely over the mouth so as to hide it from view, and their different aspects under various degrees of expansion or contraction account for the discrepancies in the description of these animals. We have seen that the Pleurobrachia moves with the mouth upward; but the Bolina, on the contrary, usually carries the mouth downward, though it occasionally reverses its position, and in this attitude, with the lobes spread open, it is exceedingly graceful in form, and looks like a white flower with the crown fully expanded. These broad lobes are balanced on the other sides of the body by four smaller appendages, divided in pairs, two on each side (Fig. 32), called auricles. These so-called auricles are in fact organs of the same kind as the larger lobes, though less developed. The rows of locomotive flappers on the Bolina differ in length from each other (Fig. 31), instead of being equal, as in the Pleurobrachia. The four longest ones are opposite each other on those sides of the body where the larger lobes are developed, the four short ones being in pairs on the sides where the auricles are placed. At first sight they all seem to terminate at the margin of the body, but a closer examination shows that the circulating tubes connected with the longer row extend into the lobes, where they wind about in a variety of complicated involutions. (Fig. 32.) The movements of the Bolina are more sluggish than those of the Pleurobrachia, and the long tentacles, so graceful an ornament to the latter, are wanting in the former. With these exceptions the description given above of the Pleurobrachia will serve equally well for the Bolina. The structure is the same in all essential points, though it differs in the size and proportion of certain external features, and its play of color is less brilliant than that of the Pleurobrachia. The Bolina, with its slow, undulating motion, its broad lobes sometimes spreading widely, at other times folded over the mouth, its delicacy of tint and texture, and its rows of vibrating fringes along the surface, is nevertheless a very beautiful object, and well rewards the extreme care without which it dies at once in confinement.

_Idyia_. (_Idyia roseola_ AG.)

The lowest genus of Ctenophoræ found on our coast, the Idyia (Fig. 33), has neither the tentacles of the Pleurobrachia, nor the lobes of the Bolina. It is a simple ovate sphere, the interior of which is almost entirely occupied by an immense digestive cavity. It would seem that the reception and digestion of food is intended to be the almost exclusive function of this animal, for it has a mouth whose ample dimensions correspond with its capacious stomach. Instead of the longitudinal split serving as a mouth, in the Bolina and Pleurobrachia, one end of the body in the Idyia is completely open (Fig. 33), so that occasionally some unsuspicious victim of smaller diameter than itself may be seen to swim into this wide portal, when suddenly the door closes behind him with a quick contraction, and he finds himself a prisoner. The Idyia does not always obtain its food after this indolent fashion however, for it often attacks a Bolina or Pleurobrachia as large or even larger than itself, when it extends its mouth to the utmost, slowly overlapping the prey it is trying to swallow by frequent and repeated contractions, and even cutting off by the same process such portions as cannot be forced into the digestive cavity.

The general internal structure of the Idyia corresponds with that of the Bolina and Pleurobrachia; it has the same tubes branching horizontally from the main cavity, then ramifying as they approach the periphery till they are multiplied to eight in all, each of which gives off one of the vertical tubes connected with the eight rows of locomotive flappers. Opposite the mouth is the eye-speck, placed as in the two other genera, at the centre of a small circumscribed area, which in the Idyia is surrounded by delicate fringes, forming a rosette at this end of the body. These animals are exceedingly brilliant in color; bright pink is their prevailing hue, though pink, red, yellow, orange, green, and purple, sometimes chase each other in quick succession along their locomotive fringes. At certain seasons, when most numerous, they even give a rosy tinge to patches on the surface of the sea. Their color is brightest and deepest before the spawning season, but as this advances, and the ovaries and spermaries are emptied, they grow paler, retaining at last only a faint pink tint. They appear early in July, rapidly attain their maximum size, and are most numerous during the first half of August. Toward the end of August they spawn, and the adults are usually destroyed by the early September storms, the young disappearing at the same time, not to be seen again till the next summer. It is an interesting question, not yet solved, to know what becomes of the summer's brood in the following winter. They probably sink into deep waters during this intervening period. The Idyia, like the Pleurobrachia, moves with the mouth upward, but inclined slightly forward also, so as to give an oblique direction to the axis of the body.[4]

[Footnote 4: Until this summer only the three genera of Ctenophoræ
above mentioned were supposed to exist along our coast, but during
the present season I have had the good fortune to find two
additional ones. One of them, the Lesueuria, resembles a Bolina
with the long lobes so cut off, that they have a very stunted
appearance in comparison with those of the Bolina. The other, the
Mertensia, is closely allied to Pleurobrachia; it is exceedingly
flattened and pear-shaped. This species was discovered long ago by
Fabricius, but had escaped thus far the attention of other
naturalists. (_A. Agassiz_.)]

EMBRYOLOGY OF CTENOPHORÆ.

All the Ctenophoræ are reproduced from eggs, these eggs being so transparent that one may follow with comparative ease the changes undergone by the young while still within the egg envelope. Unfortunately, however, they are so delicate that it is impossible to keep them alive for any length of time, even by supplying them constantly with fresh sea-water, and keeping them continually in motion, both of which are essential conditions to their existence. It is therefore only from eggs accidentally fished up at different stages of growth that we may hope to ascertain any facts respecting the sequence of their development. When hatched, the little Ctenophore is already quite advanced. It is small when compared with the size of the egg envelope, and long before it is set free, it swims about with great velocity within the walls of its diminutive prison (Fig. 35). The importance of studying the young stages of animals can hardly find a better illustration than among the Ctenophoræ. Before their extraordinary embryonic changes were understood, many of the younger forms had found their way into our scientific annals as distinct animals, and our nomenclature thus became burdened with long lists of names which will disappear as our knowledge advances.

The great size of their locomotive flappers in proportion to the rest of the body, is characteristic of the young Ctenophoræ. They seem like large paddles on the sides of these tiny transparent spheres, and, owing to their great power as compared with those of the adult, the young move with extraordinary rapidity. The Pleurobrachia alone retains its quickness of motion in after life, and although its long graceful streamers appear only as short stumpy tentacles in the young (Fig. 34), yet its active little body would be more easily recognized in the earlier stages of growth than that of the other Ctenophoræ. Figs. 34, 35, and 36 show the Pleurobrachia at various stages of growth; Fig. 34, with its thick stunted tentacles and short rows of flappers, is the youngest; the flappers themselves are rather long at this age, looking more like stiff hairs than like the minute fringes of the adult. In Fig. 35 the tentacles are already considerably longer and more delicate; in Fig. 36 the vertical tubes are already completed, while Figs. 27-29 present it in its adult condition.

The Idyia differs greatly in appearance at different periods of its development, and, indeed, no one would suspect, without some previous knowledge of its transformations, that the young Idyia, with its rapid gyrations, its short ambulacral tubes, like immense pouches (Fig. 37), its large pigment spots scattered over the surface (Fig. 38), was an earlier stage of the rosy-hued Idyia, which glides through the water with a scarcely perceptible motion. Figs. 37-40 represent the various stages of its growth. It will be seen how very short are the locomotive fringes (Fig. 39) in comparison with those of the full-grown ones (Fig. 33). It is only in the adult Idyia that these rows attain their full height, and the tubes, ramifying throughout the body (Fig. 40), are completed.

The Bolina, in its early condition, recalls the young Pleurobrachia. At this period it has the same rapid motion, and when somewhat more advanced, long tentacles, resembling those of the Pleurobrachia, make their appearance (Fig. 41); it is only at a later period that the tentacles become contracted, while the large lobes (Fig. 42), so characteristic of Bolina, are formed by the elongation of the oral end of the body, the auricles becoming more conspicuous at the same time (Fig. 43). A little later the lobes enlarge, the movements become more lazy; it assumes both in form and habits the character of the adult Bolina.

The series of changes through which the Ctenophoræ pass are as remarkable as any we shall have occasion to describe, though not accompanied with such absolutely different conditions of existence. The comparison of the earlier stages of life in these animals with their adult condition is important, not only with reference to their mode of development, but also because it gives us some insight into the relative standing of the different groups, since it shows us that certain features, permanent in the lower groups, are transient in the higher ones. A striking instance of this occurs in the fact mentioned above, that though the long tentacles so characteristic of the adult Pleurobrachia exist in the young Bolina, they yield in importance at a later period to the lobes which eventually become the predominant and characteristic feature of the latter.

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DISCOPHORÆ.

The disk of the Discophoræ is by no means so delicate as that of the other Jelly-fishes. It seems indeed quite solid, and somewhat like cartilage to the touch, and yet so large a part of its bulk consists of water, that a Cyanea, weighing when alive about thirty-four pounds, being left to dry in the sun for some days, was found to have lost about 99/100 of its original weight,--only the merest film remaining on the paper upon which it had been laid. The prominence of the disk in this group of Jelly-fishes is well characterized by their German name, "Scheiben quallen," viz. disk-medusæ. We shall see hereafter that the disk, so large and seemingly solid in the Discophoræ, thins out in many of the other Jelly-fishes, and becomes exceedingly concave. This is especially the case in many of the Hydroid Medusæ, where it assumes a bell-shaped form, and is constantly spoken of as the bell. It should be remembered, however, in reading descriptions of these animals, that the so-called bell is only a modified disk, and perfectly homologous with that organ in the Discophoræ.

The Discophorous Medusæ are distinguished from all others by the peculiar nature of the reproductive organs. They are contained in pouches (Fig. 50, _o_, _o_, _o_, _o_), the contents of which are first discharged into the main cavity, and then pass out through the mouth. Pillars support the four angles of the digestive cavity, thus separating the lower from the upper floor of the disk, while the chymiferous tubes (Fig. 50) branch and run into each other near the periphery, forming a more or less complicated anastomosing network, instead of a simple circular tube, as is the case with the Hydroid Medusæ. (Fig. 74.)

_Cyanea_. (_Cyanea arctica_ PÉR. et LES.)

In our descriptions of the Discophoræ, we may give the precedence to the Cyanea on account of its size. This giant among Jelly-fishes is represented in Fig. 44. It is much to be regretted that these animals, when they are not so small as to escape attention altogether, are usually seen out of their native element, thrown dead or dying on the shore, a mass of decaying gelatinous matter. All persons who have lived near the sea are familiar with the so-called Sea-blubbers, sometimes strewing the sandy beaches after the autumn storms in such numbers that it is difficult to avoid them in walking or driving. In such a condition the Cyanea is far from being an attractive object; to form an idea of his true appearance, one must meet him as he swims along at midday, rather lazily withal, his huge semi-transparent disk, with its flexible lobed margin, glittering in the sun, and his tentacles floating to a distance of many yards behind him. Encountering one of those huge Jelly-fishes, when out in a row-boat one day, we attempted to make a rough measurement of his dimensions upon the spot. He was lying quietly near the surface, and did not seem in the least disturbed by the proceeding, but allowed the oar, eight feet in length, to be laid across the disk, which proved to be about seven feet in diameter. Backing the boat slowly along the line of the tentacles, which were floating at their utmost extension behind him, we then measured these in the same manner, and found them to be rather more than fourteen times the length of the oar, thus covering a space of some hundred and twelve feet. This sounds so marvellous that it may be taken as an exaggeration; but though such an estimate could not of course be absolutely accurate, yet the facts are rather understated than overstated in the dimensions here given. And, indeed, the observation was more careful and precise than the circumstances would lead one to suppose, for the creature lay as quietly, while his measure was taken, as if he had intended to give every facility for the operation. This specimen was, however, of unusual size; they more commonly measure from three to five feet across the disk, while the tentacles may be thirty or forty feet long. The tentacles are exceedingly numerous (see Fig. 44), arising in eight distinct bunches, from the margin of the disk, and hanging down in a complete labyrinth.

These animals are not so harmless as it would seem, from their soft, gelatinous consistency; it is no pleasant thing when swimming or bathing to become entangled in this forest of fine feelers, for they have a stinging property like nettles, and may render a person almost insensible, partly from pain, and partly from a numbness produced by their contact, before he is able to free himself from the network in which he is caught. The weapons by which they produce these results seem so insignificant, that one cannot but wonder at their power. The tentacles are covered by minute cells, lasso-cells as they are called, (similar to those of Astrangia, Fig. 19,) each one of which contains a whip finer than the finest thread, coiled in a spiral within it.

These myriad whips can be thrown out at the will of the animal, and really form an efficient galvanic battery. Behind the veil of tentacles, and partly hidden by it, four curtains, with lobed or ruffled margins, dimly seen in Fig. 44, hang down from the under surface of the disk. The ovaries are formed by four pendent pouches, placed near the sides of the mouth, and attached to four cavities within the disk. Around the circumference of the disk are eight eye-specks, each formed by a small tube protected under a little lappet or hood rising from the upper surface of the disk. The prevailing color of this huge Jelly-fish is a dark brownish-red, with a light, milk-white margin, tinged with blue, the tentacles and other pendent appendages having a somewhat different hue from the disk. The ovaries are flesh-colored, the curtain formed by the expansion of the lobes of the mouth is dark brown, while the tentacles are of different colors, some being yellow, others purple, and others reddish brown or pink.

Strange to say, this gigantic Discophore is produced by a Hydroid measuring not more than half an inch in height when full grown; could we follow the history of any egg laid by one of these Discophoræ in the autumn, and this has indeed been partially done, we should see that, like any other planula, the young hatched from the egg is at first spherical, but presently becomes pear-shaped, and attaches itself to the ground. From the upper end tentacles project (see Fig. 45), growing more numerous, as in Fig. 46, though they never exceed sixteen in number. As it increases in height constrictions take place at different distances along its length, every such constriction being lobed around its margin, till at last it looks like a pile of scalloped saucers or disks strung together (see Fig. 47). The topmost of these disks falls off and dies; but all the others separate by the deepening of the constrictions, and swim off as little free disks (Fig. 48), which eventually grow into the enormous Jelly-fish described above. These three phases of growth, before the relation between them was understood, have been mistaken for distinct animals, and described as such under the names of Scyphistoma, Strobila, and Ephyra.

_Aurelia_. (_Aurelia flavidula_ PÉR. et LES.)

Another large Discophore, though by no means to be compared to the Cyanea in size, is our common Aurelia (Figs. 49, 50) Its bluish-white disk measures from twelve to fifteen inches in diameter, but its dimensions are not increased by the tentacles, which have no great power of contraction and expansion, and form a short fringe around its margin, widening and narrowing slightly as the tentacles are stretched or drawn in. It is quite transparent, as may be seen in Fig 49, where all the fine ramifications of the chymiferous tubes as well as the ovaries, are seen through the vault of the disk. Fig. 50 represents the upper surface, with the ovaries around the mouth, occupying the same position as those of the Cyanea, though they differ from the latter in their greater rigidity, and do not hang down in the form of pouches. The males and females in this kind of Jelly-fish may be distinguished by the difference of color in the reproductive organs, which are rose-colored in the males, and of a dull yellow in the females. The process of development is exactly the same in the Aurelia as in the Cyanea, though there is a very slight difference in their respective Hydroids. They are, however, so much alike, that one is here made to serve for both, the above figures being taken from the Hydroid of the Aurelia. It is curious, that while, as in the case of the Aurelia and Cyanea, very dissimilar Jelly-fishes may arise from almost identical Hydroids, we have the reverse of the proposition, in the fact that Hydroids of an entirely distinct character may produce similar Jelly-fishes.

The embryos or little planulæ, hatched from the Cyanea and Aurelia in the fall, seem to be gregarious in their mode of life, swimming about together in great numbers till they find a suitable point of attachment, and assume their fixed Hydroid existence. The Cyaneæ, however, when adult, are usually found singly, while the Aureliæ, on the contrary, seek each other, and commonly herd together.

_The Campanella_. (_Campanella pachyderma_ A. AG.)

The Campanella (Fig. 51) is a pretty little Jelly-fish, not larger than a pin's head, reproduced directly from eggs, without passing through the Hydroid stage. During its early stages of growth it probably remains attached to floating animals, thus leading a kind of parasitic existence; but as its habits are not accurately known, this cannot be asserted as a constant fact respecting them. The veil in this Jelly-fish is very large, forming pendent pouches hanging from the circular canal (see Fig. 51), and leaving but just room enough for the passage of the proboscis between the folds. It may not be amiss to introduce here a general account of this organ, which occurs in many of the Medusæ, though it has very different proportions in the various kinds. It is a delicate membrane, hanging from the circular tube, so as partially to close the mouth of the bell, leaving a larger or smaller opening for the passage of the water, which is taken in and forced out again by the alternate expansions and contractions of the bell.

There are but four chymiferous tubes in the Campanella, and four stiff tentacles, which in consequence of the peculiar character of the veil appear, when the animal is seen in profile, to start from the middle of the disk. The ovaries consist of eight pouches, placed near the point of junction of the four chymiferous tubes. (Fig. 52.) This little Medusa is of a dark yellowish color with brownish ocellated spots, scattered profusely over the upper part of the disk.

_Circe_. (_Trachynema digitale_ A. AG.)

Among the Jelly-fishes, the position of which is somewhat doubtful, is the Circe (Fig. 53), differing greatly in outline from the ordinary Jelly-fishes. As may be seen in Figure 53, the bell forms but a small portion of the animal; it rises in a sharp cone on the summit, thinning out at the lower edge, to form the large cavity in which hangs the long proboscis and the eight ovaries, four of which may be seen in Fig. 53 crowded with eggs. The Circe differs in consistency, as well as in form, from other Jelly-fishes. It is hard and horny to the touch, and the veil, usually so light and filmy, is here a thick folded membrane, which at every stroke of the animal forces the water in and out of the cavity. It is very active, moving by powerful jerks, each one of which throws it far on its way. It advances usually in straight lines; or, if it wishes to change its direction, it drives the water out of the veil suddenly on one side or the other, so as to shoot off, sometimes at right angles with its former path. Four large pedunculated eyes, hidden in the figure by the tentacles, stand out prominently from the circular tube. When the animal is in motion, the tentacles are carried closely curled around the edge of the disk, as in Fig. 53, where the Circe is represented under a magnifying power of two and a half diameters. This Jelly-fish is of a delicate rose color, the tentacles assuming, however, a dark-purple tint at their extremities when contracted.

_Lucernaria_. (_Haliclystus auricula_ CLARK.)

One of the prettiest and most graceful, as well as one of the most common of our Jelly-fishes, is the Lucernaria (Fig. 54). It has such an extraordinary contractility of all its parts, that it is not easy to describe it under any definite form or position, since both are constantly changing; but perhaps of all its various attitudes and outlines none are more normal to it than those given in Fig. 54. It frequently raises itself in the upright position represented here by the individual highest on the stem, spreading itself in the form of a perfectly symmetrical cup or vase, the margin of which is indented by a succession of inverted scallops, the point of junction between every two scallops being crowned by a tuft of tentacles. But watch it for a while, and the sides of this vase turn backward, spreading completely open, till they present the whole inner surface, with the edges even curved a little downward, drooping slightly, and the proboscis rising in the centre. In such an attitude one may trace with ease the shape of the mouth, the lobes surrounding it, as well as the tubes and cavities radiating from it toward the margin. A touch is, however, sufficient to make it close upon itself, shrinking together in the attitude of the third individual in Fig. 54, or even drawing its tentacles completely in, and contracting all its parts till it looks like a little ball hanging on the stem. These are but a few of its manifold changes, for it may be seen in every phase of expansion and contraction. Let us now look for a moment at the details of its structure. The resemblance to a cup or vase, as in the upper figure of the wood-cut (Fig. 54), is deceptive; for a vase is hollow, whereas the Lucernaria, though so delicate and transparent that its upper surface, when thus stretched, seems like a mere film, is nevertheless a solid gelatinous mass, traversed by certain channels, cavities, and partitions, but otherwise continuous throughout. The peduncle by which it is attached is but an extension of the floor of a gelatinous disk, corresponding to that of any Jelly-fish. Four tubes pass through the whole length of this peduncle, and open into four chambers, dividing the digestive cavity above into as many equal spaces. (Fig. 55.) These spaces are produced by folds in the upper floor of the disk, uniting it to the lower floor at given distances, and forming so many partition-walls, dividing the digestive sac into four distinct cavities. These lines of juncture between the two floors, where the partitions occur, produce the four radiating lines, running from the proboscis to the margin of the disk, on the upper surface. (Fig. 55.) The triangular figures, running from the mouth to each cluster of tentacles, are produced by the ovaries, which consist of distinct pouches or bags attached to the upper surface of the disk, and hanging down into the cavities below; every little dot within these triangular spaces represents such a bag. Each bag is crowded with eggs, which drop into the digestive cavity at the spawning season, and are passed out at the mouth. The tentacles always grow in clusters, but are nevertheless arranged according to a regular order. They are club-shaped at their extremities, but are hollow throughout, opening into the chambers of the digestive cavity, two of the clusters thus being connected with each chamber. Their chief office seems to be to catch the food and convey it to the mouth, though they may also be used as a kind of suckers, and the animal not unfrequently attaches itself by means of these appendages. Between every two clusters of tentacles will be observed a short, single appendage, of an entirely different appearance. These are the so-called auricles, and though so unlike tentacles in the adult animal, when in their earlier stages (Fig. 56) they resemble each other closely. But as their development goes on, the tentacles stretch out into longer, more delicate flexible organs, while the auricles remain short and compact throughout life. They contain a slight pigment spot representing an eye, though how far it serves the purpose of vision remains doubtful. They are chiefly used by the animal as a means of adhering to any surface upon which it may wish to fasten itself; for the Lucernaria, though usually found attached to eel-grass in shoal water, has the power of independent motion, and frequently separates from its resting-place, floating about freely in the water for a while, or attaching itself anew by means of the auricles and tentacles upon some other object. The color of this pretty Acaleph varies from a greenish hue to green, with a faint tinge of red, or to a reddish brown. One of its commonest and most exquisite tints is that of a pale aqua-marine. It may be found along our shores wherever the eel-grass grows, and as far out as this plant extends. It thrives admirably in confinement, and for this reason is especially adapted to the aquarium.

* * * * *

HYDROIDS.

Under this order, the general character of which has already been explained in the introductory chapter on Acalephs, are included a number of groups which, whether as Hydroid communities in their earlier phases of existence, or as free swimming Medusæ in their farther development, challenge our admiration, both for their beauty of form and color, and their grace of motion. Some of them are so minute that they escape the observation of all but those who are laboriously seeking for the hidden treasures of the microscopic world, but the greater number are large enough to be readily found by the most inexperienced collector, when his attention is once drawn to them; and he may easily stock his aquarium with these pretty little communities, and even trace the development of the Jelly-fishes upon them.

To the Hydroids belong the Campanularians, the Sertularians, and the Tubularians. Some examples of each, as represented on our shores, will be found under their different heads, accompanied with full descriptions. There is another group usually considered as distinct from Hydroids, and known as a separate order among Acalephs, under the name of Siphonophoræ, but included with them here in accordance with the views of Vogt, Agassiz, and others, in whose opinion they differ from the ordinary Hydroid communities only in being free and floating, instead of fixed to the ground. Some new facts, published here for the first time, tend to sustain the accuracy of this classification.[5] With these few preliminary remarks to show the connection of the order, let us now look at some of the animals belonging to it more in detail.

[Footnote 5: See Chapter on Nanomia.]

_Campanularians_.

All the Campanularians, of which Oceania (Fig. 68), Clytia (Fig. 73), and Eucope (Fig. 61) form a part, belong among those little shrub-like communities of animals called Hydroids, from which most of our Jelly-fishes are developed. They differ in one essential feature from the Tubularians. (Fig. 93.) The whole stem, from summit to base, is enveloped in a horny sheath, extending around both the fertile and sterile individuals of the community, and forming a network at the base of the stem, which serves as a kind of foundation for the whole stock. To the naked eye such a community looks like a tiny shrub (see Fig. 57), with the branches growing in regular alternation on either side of the stems. The reproductive calycles, i.e. the protecting envelopes covering the young Medusæ, usually arise in the angles of the branches formed by a prolongation of the sheath. These calycles or bells, as they are called, assume a great variety of shapes,--elliptical, round, pear-shaped, or ringed like the Clytia. (Fig. 72.) In one such bell there may be no less than twenty or thirty Medusæ developed one below the other; when ready to hatch, the calycle bursts and allows them to escape.

_Eucope_. (_Eucope diaphana_ AG.)

In Figs. 60 and 61 we have a representation of our little Eucope, one of the prettiest of the Jelly-fishes belonging to this group; Fig. 57 represents the Hydroid from which it arises; a single branch with the reproductive bell being magnified in Fig. 58. In Fig. 59 is seen a portion of the Jelly-fish disk, with the fringe of tentacles highly magnified. The disk of the Eucope (Fig. 60) looks like a shallow bell, of which the proboscis often seems to form the handle; for the disk has such an extraordinary thinness that it turns inside out with the greatest ease, so that the inner surface may become at any moment the outer one, with the proboscis projecting from it, as in Fig. 60, while the next movement of the animal may reverse its whole position, and the proboscis then hangs down from the inside, as in other Jelly-fishes. (See Fig. 61.)

The tentacles are solid and stiff like little hairs, and two of them, in each quarter-segment of the disk, have small concretions at the base, which are no doubt eye-specks. (See Fig. 62.) Along the chymiferous tubes little swellings are developed, which increase gradually, and become either ovaries or spermaries, according to the sex of the animal. (Fig. 63.) In the adult the genital organs hang down, like elongated bags, from the chymiferous tubes. (Fig. 64.) The tentacles are numerous, multiplying to about a hundred and ninety-two in the adult, and increasing according to the numerical law to be explained in the description of the Oceania.

This little Jelly-fish is one of the most common in our Bay. There is not a night or day when they cannot be taken in large numbers, from the early spring till late in the autumn; and as the breeding season lasts during the whole of that period, they are found in all possible stages of growth. In consequence of this, the course of their development, and the relation between the different phases of their existence as Hydroids, and afterwards as Acalephs, are well known, though the successive steps of their growth have not been traced connectedly, as in some of the other Jelly-fishes, the Tima or Melicertum, for instance. The process is, however, so similar throughout the class of Hydroids, that, having followed it from beginning to end in some of the groups, we have the key to the history of others, whose development has not been so fully traced. The eggs laid by the Eucope in the autumn develop into planulæ, which acquire their full size as Hydroid communities toward the close of the winter, and the development of the young Medusæ upon them, as described above, begins with the opening spring.

_Oceania_. (_Oceania languida_ A. AG.)

The Oceania (Fig. 68) is so delicate and unsubstantial, that with the naked eye one perceives it only by the more prominent outlines of its structure. We may see the outline of the disk, but not the disk itself; we may trace the four faint thread-like lines produced by the radiating tubes traversing the disk from the summit to the margin; and we may perceive, with far more distinctness, the four ovaries attached to these tubes near their base; we may see also the circular tube uniting the radiating tubes, and the tentacles hanging from it, and we can detect the edge of the filmy veil that fringes the margin of the disk. But the substance connecting all these organs is not to be distinguished from the element in which it floats, and the whole structure looks like a slight web of threads in the water, without our being able to discern by what means they are held together. Under the microscope, however, the invisible presently becomes visible, and we find that this Jelly-fish, like all others, has a solid gelatinous disk.