« AnteriorContinuar »
fleshy lining, continuous with the endocyst ; this internal tunic has numerous large, round, nucleated cells distributed through its substance, and internally it presents an uneven surface. The two tunics of the gemma are to become the ectocyst and endocyst of the future cell.
By this time the gemma has become considerably elongated and has acquired a clavate form, and its cavity begins to be cut off from that of the parent-cell by the formation of a septum. We next perceive that a portion of the lining tunic, near the wide extremity of the gemma, projects in the form of a roundish mass into the interior of the young cell (fig. 4). It is the rudimental polypide, and we soon find in it a cavity, which is to become that of the tentacular sheath, within which, when viewed in front, may be seen an oval ring (fig. 5) which is afterwards to become the lophophore of the polypide. This ring is at first quite simple, resembling a mere fold of thickish membrane, but in a short time it presents all round a series of minute tubercles (fig. 6), the rudiments of the future tentacula. When viewed laterally (fig. 7), the lophophore in this stage is seen to be decidedly bilateral, being prolonged towards the side where the rectum is to make its appearance. The central space between the rudimental tentacula is prolonged downwards, constituting the first trace of a pharynx ; and immediately below this, the mass of the polypide is hollowed out into an internal cavity, which is to become stomach and intestine. This cavity is at first filled with clear, round bodies, having a high refracting power, but in which I could not trace the double outline of a true cell-wall. The polypide is now seen to be suspended from the wall of the cell by a membranous sac. This sac is closely adherent to the polypide behind the lophophore, but upon the lophophore and tentacula it has become free, and here constitutes the tentacular sheath. Some delicate fibres, the rudiments of the retractor of the polypide, may now be plainly seen extending from just behind the lophophore to the walls of the cell. The body of the polypide at the spot directly opposite to the rudimental crown of tentacula, is also seen to be connected by a short, thick, fleshy mass, with the walls of the cell. Circular fibres may have been already seen in the lining membrane of the cell; these are chiefly collected near its proximal end (fig. 6), and are to become the parietal muscles of the adult. The body of the polypide continues to elongate itself (fig. 8), and we can soon distinguish in its cavity the three regions of esophagus, stomach, and intestine. The rudimentary parieto-vaginal muscles may also be seen extending from the tentacular sheath to the walls of the cell; the thick, fleshy mass, by which the body of the polypide was connected with the cell-walls, has become lengthened and divided into two chords, the anterior and posterior funiculus ; the tentacula continue to increase in length ; the lophophore loses its bilaterality and becomes orbicular, and we now (fig. 9) find little wanting to give to the polypide and its cell the form presented by the adult.
Up to this period the young polypide has been entirely shut off from all communication with the external water, and its nutrition must have been effected through the general nutrition of the colony; now, however, an opening occurs in the new cell just over the tentacular crown of the polypide, and the last stage of development is entered on. The tentacular crown rapidly acquires its complete form, the inferior extremity of the alimentary canal becomes elongated into the great cul-de-sac of the stomach, the muscles are by this time all formed, and the polypide is capable of exsertion and retraction. It is now no longer dependent for its growth on the general nutrition of the colony, but has become an independent zooid, obtaining its food from without, and submitting it to the action of its own digestive system.
The development of the bud in the species with crescentic lophophore is essentially the same as that just described. The first appearance of the bud in Lophopus is seen near the orifice of the cell, as a minute spherical tubercle (Pl. XI, fig. 10), projecting from the inner surface of the endocyst into the perigastric space. It is evidently composed of a mass of minute cells with a more condensed peripheral layer. It now increases in size, and a clear, oval space becomes apparent towards one side of it (fig. 11). This space soon acquires a triangular figure (fig. 12), and it is now evident that the lophophore has begun to develop itself from its walls, and that the triangular space corresponds to the interval between the two arms of the rudimental lophophore. By this time the young polypide has become removed from the surface of the endocyst of the parent cell, to which it is now only connected by a membranous sac which suspends it in the perigastric space of the adult, and which ultimately becomes the tentacular sheath. At the same time, the alimentary canal begins to appear as a minute cavity in the part of the bud farthest removed from the walls of the cell. This cavity increases in size (figs. 13, 14), and becomes the stomach, while the intestine also shows itself as a tubular process on one side of the bud (fig. 13). The lophophore, which from its earliest appearance is distinctly bilateral, now presents the form of a semicircle, with a bilobed base, and thickened margin ; as yet, there is no trace of tentacula. The retractor muscles show themselves as two fasciculi passing off from the base of the lophophore, and the funiculus may also be seen attached to the posterior extremity of the body. The tentacula now make their appearance as minute tubercles upon the thickened margin of the lophophore (fig. 15). The tentacula and arms of the lophophore are then simultaneously elongated (fig. 16), while the three regions of the alimentary canal, esophagus, stomach, and intestine, are easily distinguishable, and little more is now wanting than an opening through the walls of the parent-cell into the tentacular sheath of the young polypide, to complete the development of the gemma.
In all the fresh water phylactolæmatous Polyzoa, bodies of a very peculiar nature occur at certain seasons, lying loose in the perigastric space. To these bodies, for reasons to be presently mentioned, I propose to give the name of Statoblasts. From the earliest period that the fresh-water Polyzoa became an object of study, the statoblasts attracted the attention of observers. Their form is not exactly the same in the different species, but they may be generally described as lenticular bodies, varying, according to the species, from an orbicular to an elongated oval figure, and enclosed in a horny shell, which consists of two concavo-convex discs united by their margins, where they are further strengthened by a ring which runs round the entire margin, and is of a different structure from the disc (Pl. I, figs. 3–8; Pl. IV, figs. 4, 5; &c.) In Fredericella alone is the marginal ring so little developed as to be scarcely apparent. In all, one side would seem to be slightly more convex than the other. In all cases, except Cristatella and Pectinatella, the statoblast is destitute of
further appendage ; in these two genera, however, it is furnished, when mature, with hooked spines, which, at least in Cristatella, spring alternately from the two sides just within the annulus (Pl. I, figs. 4—7), being generally more numerous and better developed on the more convex side; they thence pass outwards over the annulus, and project in short rays beyond the margin. The disc in all the species is of a deep brown colour, and would seem to be composed of a single layer of hexagonal cells, whose external walls in most cases slightly project beyond the surface of the disc, and thus give to the latter an elegantly mammillated condition. In some cases, however, the cellular condition of one or both discs is very obscure. The annulus is also composed of cells, which here occur in several layers; these cells are also for the most part larger than those of the disc and of a different colour; they are filled with air, giving to the annulus a light spongy texture, and act as a float by which the statoblast, when set free, is kept near the surface of the surrounding water.
When the statoblasts are placed under circumstances favouring their development, they open by the separation from one another of the two faces, and there then escapes from them a young polyzoon already in an advanced stage of development, and in all essential points resembling the adult individual in whose cell the statoblasts were produced (Pl. XI, figs. 42, 43).
The statoblasts have been always viewed and described as the eggs of the polyzoon in whose cell they occur, a very natural mistake, and one the more excusable as the true ovary had not yet been detected. Into this error I fell myself;* but I have now become convinced that they are a peculiar form of bud, and must on no account be confounded with genuine ova.
They are produced in the funiculus, from which they are evidently developed as buds, and may generally be seen in various stages of growth, arranged upon this chord like beads on a necklace, being younger as they approach the distal extremities of the funiculus (Pl. III, fig. 7; V, fig. 5, 2; XI, fig. 36).
In Lophopus, I have succeeded in following them through various stages of their early development. Their first appearance here is in the form of a little swelling upon the funiculus, consisting of a mass of minute cells, surrounded by a denser layer, which is continuous with the surface of the funiculus. The swelling now increases in size, and assumes a more regularly oval form, while its contents appear more uniformly granular, and are plainly seen to be composed of two masses in close apposition with one another (fig. 37). We next find that the two masses have lost their distinctness, and become fused together, and the whole contents now appear to be composed of minute cells confined by an external, common, transparent membrane, which is itself manifestly cellular (fig. 38). This cellular condition of the contents must not be confounded with a true segmentation. The whole body now assumes a more lenticular form, and within the external envelope two other investments begin to show themselves. One of these (the more internal) extends over the whole surface of the cellular contents, but the other is confined to the margin of the lenticular mass which it embraces in the form of a ring (fig. 39). No manifest structure, beyond a simply granular one, can be as yet detected in either of these last-formed envelopes; but the ring is soon seen to be composed of distinct cells (fig. 40), which present a bright central nucleus-like point, and a number of concentric layers, which remind us of the secondary deposits in certain vegetable cells. Up to this point the investments are all colourless, and nearly transparent; but we now find that the internal envelope and annulus become more and more opaque, while the former assumes a deep brown colour, and the latter becomes yellow. They have both acquired a horny consistence,
and the annulus is composed of large hexagonal cells filled with air. If the whole be now crushed, under the microscope, multitudes of cells will escape, all filled with minute, strongly refracting corpuscles (fig. 41, 414, 41'); but any further observation of the progressive development of the contents, up to the period of the opening of the statoblast and the final escape of a young polyzoon, is henceforth, in consequence of the opacity of the covering, impossible. The statoblast has now acquired the complete form characteristic of the species, and, breaking loose from the funiculus, it falls free into the perigastric space, still surrounded by the delicate external transparent membrane, which is soon torn and disappears.
When exposed to conditions favorable for its further development, the two faces, after a longer or shorter period, separate from one another, as has been already said, and a young polyzoon gradually emerges and floats away freely through the surrounding water; this phenomenon I have observed in Cristatella, and several species of Plumatella (figs. 42, 43). The surface of the little polyzoon thus become free is destitute of cilia, except on the tentacula ; and the motions of the young animal seem to be quite passive, except so far as they may be influenced by the ciliary action of the tentacula. At the period of its escape
it the essential organization of the adult; the retractor muscles are well developed, and the polypide is capable of regular exsertion and retraction ; but the endocyst is colourless and transparent, and free from the earthy particles which in the greater number of species are afterwards formed in it, and the little animal is still sirnple. It loses no time, however, in developing gemmæ, which soon change it to the compound form of the adult. In many cases the two separated faces of the statoblast continue for some time to adhere to the posterior end of the young polyzoon, like the valves of a bivalve shell.
In Cristatella the essential stages of the development of the statoblast are similar to those just described in Lophopus, but the external envelope acquires here over its whole surface minute cilia (Pl. I, fig. 3), and becomes separated from the rest of the statoblast by a considerable space, which is filled with semi-fluid granular contents. The statoblast acquires its full size still surrounded by the ciliated envelope, but as yet no trace of the spines is visible; these, however, shortly after show themselves growing out from the two faces of the statoblast (fig. 4); they penetrate the granular matter included within the external investment, and soon impinge on the last-mentioned membrane (fig. 5), which by this time has lost its cilia, and which now gives way, torn by the grapple-like extremities of the spines. The external and granular investments now rapidly disappear, and the statoblast presents itself as the elegant little spiny lenticular body (fig. 6) so characteristic of the genus Cristatella.
I have sought in vain, in all the fresh-water Polyzoa, for some orifice through which the statoblasts or ova may escape from the cells ; and yet, from the large size and incompressible nature of the former, such an orifice, were it present, could hardly escape detection. Meyen, it is true, states that he has witnessed in Alcyonella fungosa the escape of an egg through an opening in the vicinity of the anus; but, notwithstanding a similar observation already noticed as made by Van Beneden on the marine Laguncula repens, this I feel certain has been an imperfect observation of Meyen, and that the escape of the egg was the result of some accidental laceration of the tissues in this spot. There is, then, no natural aperture through which either ova or statoblasts can escape, and their liberation, I am convinced, can only take place after the destruction of the soft parts of the polyzoon has afforded to them a mode of egress through the orifice of the cell.
**Naturgeschichte der Polypen,' Isis, 1828.
In two species of fresh-water Polyzoa, Plumatella emarginata and Alcyonella Benedeni, I have observed, besides the ordinary statoblasts, another kind which is characterised by some peculiarities. In both these Polyzoa the cells may be observed towards the end of summer loaded with statoblasts which lie loose within them. These are the ordinary ones, and, in the two species of Polyzoa now under consideration, are of an elongated oval figure, with a largely developed annulus which overlaps a considerable portion of the disc (Pl. IV, figs. 7–9; VII, figs. 7, 8). But, besides these bodies, others (Pl. IV, fig. 10; VII, fig. 9) may be observed which never lie loose in the cell, but are invariably attached to the internal surface of the walls, to which they adhere by means of a peculiar cement in which no trace of structure can be detected. These differ also from the unattached statoblasts in shape, being much broader in proportion to their length, while the annulus is exceedingly narrow and presents but slight traces of that highly developed cellular structure so remarkable in the others. After the decay of the concecium many of these attached statoblasts may be seen adherent to the stone or other body on which the specimen had developed itself, and to which they are now connected in lines (Pl. VII, figs. 5, 10) through the medium of a portion of the old cell in which they had been produced. I am unable to state whether the origin and destination of the last-described bodies is similar to that of the others, and I have not succeeded in witnessing the escape from them of the young.
In Alcyonella fungosa and Lophopus crystallinus I have also occasionally seen bodies, which differ from the ordinary statoblasts of these Polyzoa by the possession of a regular elliptical aperture in the centre of their more convex face. They were always empty, and of their nature I have not been able to form any conclusion ; they are most probably abnormal.
The general structure and development of the statoblasts being now understood, the important question at once suggests itself, what is the true import of these bodies ? All that we have seen of them is manifestly in accordance with the nature of a bud. The invariable absence of germinal vesicle and germinal spot, and their never exhibiting the phenomena of yelk-cleavage, independently of the conclusive fact that true ova and ovary occur elsewhere in the same individual, are quite decisive against their being eggs. We must then look upon them as gemmæ peculiarly encysted and distined to remain for a period in a quiescent or pupa-like state. It was for this reason, therefore, that I proposed for them the name of statoblasts.*
How far the statoblasts of the Polyzoa admit of comparison with the “winter ova” of the Rotiferæ and the ephippia of Daphnia remains yet to be determined. Huxley has studied the production of “winter ova” in Lacinularia, and, though he has shown these bodies to be derived from a portion of the ovary, he is yet of opinion that they must be regarded as gemme.t He has carefully traced the early stages of their development in Lacinularia, and has shown that their contents are at one period divided into two masses. The reader will recollect that a precisely similar condition is presented by the statoblasts of the Polyzoa at an early period of their development; in the Rotiferæ, however, the two masses appear to continue distinct, while in the Polyzoa they are subsequently fused into a single mass. The recent researches
* Στατος, βλάστη.
t Quarterly Journal of Microscopical Science,' Oct., 1852.