[Footnote 2: See "Methods of Study," by Prof. Aga.s.siz.]
[Ill.u.s.tration: Fig. 26. Transverse section of a branch, showing pits, _a_ _a_ _a_ _a_, of the large Hydroids with the horizontal floors. (_Aga.s.siz_.)]
The Millepore is very abundant on the Florida reefs. From the solid base of the coral stock arise broad ridges, branching more or less along the edges, the whole surface being covered by innumerable pores, from which the diminutive animals project when expanded. (Fig. 25.) The whole ma.s.s of the coral is porous, and the cavities occupied by the Hydrae are sunk perpendicularly to the surface within the stock.
Seen in a transverse cut these tubular cavities are divided at intervals by horizontal part.i.tions (Fig. 26), extending straight across the cavity from wall to wall, and closing it up entirely, the animal occupying only the outer-most open s.p.a.ce, and building a new part.i.tion behind it as it rises in the process of growth. This structure is totally different from that of the Madrepores, Astraeans, Porites, and indeed, from all the polyp corals which, like all Polyps, have the vertical part.i.tions running through the whole length of the body, and more or less open from top to bottom.
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 cla.s.s 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 part.i.tions; or in other words, the open s.p.a.ces of the Polyps correspond to tubes in the Acalephs, while the part.i.tions in the Polyps correspond to the thick ma.s.ses 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 cla.s.ses, 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 cla.s.s, whose aliases might baffle the sagacity of a police detective. The names Acalephs, Medusae, 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 _Linnaea 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 Linnaeus after Buffon, whom he bitterly hated. Indeed, there is a world of meaning hidden under our zoological 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.
CTENOPHORae.
The Ctenophorae differ from other Jelly-fishes in their mode of locomotion. All the Discophorous Medusae, 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 a.s.similated with the food in the process of digestion, and is circulated throughout all its parts by means of ramifying tubes. In the Ctenophorae there is no such regular expansion and contraction of the disk; they are at once distinguished from the Discophorae 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 Ctenophorae 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_.)
[Ill.u.s.tration: Fig. 27. Pleurobrachia seen at right angles to the plane in which the tentacles are placed. (_Aga.s.siz_.)]
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 ill.u.s.trate 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 a.s.sume a bilateral character. This bilaterality becomes still more marked in the highest cla.s.s 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 pa.s.sing 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.
[Ill.u.s.tration: Fig. 28. Pleurobrachia seen in plane of tentacles.
(_Aga.s.siz_.)]
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 circ.u.mscribed 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 ma.s.s 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.
[Ill.u.s.tration: Fig. 29. Natural att.i.tude of Pleurobrachia when in motion.]
[Ill.u.s.tration: Fig. 30. Pleurobrachia seen from the extremity opposite the mouth.]
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 ca.n.a.l, 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 pa.s.ses into this ca.n.a.l, 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 ca.n.a.l. 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 Ctenophorae, 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 ca.n.a.l, 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 ca.n.a.l 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.)
[Ill.u.s.tration: Fig. 31. Bolina seen from the broad side; _o_ eye-speck, _m_ mouth, _r_ auricles, _v_ digestive cavity, _g_ _h_ short rows of flappers, _a_ _f_ long rows of flappers, _n_ _x_ _t_ _z_ tubes winding in the larger lobes; about half natural size.
(_Aga.s.siz_.)]
[Ill.u.s.tration: Fig. 32. Bolina seen from the narrow side; _c_ _h_ short rows of flappers, _a_ _b_ long rows of flappers; other letters as in Fig. 31. (_Aga.s.siz_.)]
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 att.i.tude, 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.)
[Ill.u.s.tration: Fig. 33. Idyia roseola seen from the broad side, half natural size; _a_ a.n.a.l opening, _b_ lateral tube, _c_ circular tube, _d_ _e_ _f_ _g_ _h_ rows of locomotive flappers.
(_Aga.s.siz_.)]
The lowest genus of Ctenophorae 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 circ.u.mscribed 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 sp.a.w.ning 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 sp.a.w.n, 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 Ctenophorae 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. Aga.s.siz_.)]
EMBRYOLOGY OF CTENOPHORae.
All the Ctenophorae 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 Ctenoph.o.r.e 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 ill.u.s.tration than among the Ctenophorae. 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 Ctenophorae. 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 Ctenophorae.
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.
[Ill.u.s.tration: Fig. 34. Young Pleurobrachia still in the egg; _t_ tentacle, _e_ eye-speck, _c_ _c_ rows of locomotive flappers, _d_ digestive cavity; greatly magnified.]
[Ill.u.s.tration: Fig. 35. Young Pleurobrachia swimming about in the egg just before hatching; magnified.]
[Ill.u.s.tration: Fig. 36. Young Pleurobrachia resembling somewhat the adult; _f_ funnel leading to a.n.a.l opening, _l_ lateral tubes, _o_ _o_ _o"_ _o"_ rows of locomotive flappers; magnified.]
[Ill.u.s.tration: Fig. 37. Young Idyia, greatly magnified; lettering as in Fig. 36; _d_ digestive cavity.]
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.