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muscles) are composed of very distinct fusiform nucleated cells, entirely resembling the musclecells of the involuntary fibre in the higher animals.
The existence of striated fibre in the Polyzoa was first noticed by Milne-Edwards, who detected it in Eschara ;* and Mr. Busk has since described and figured the same form of tissue in Anguinaria spatulata, Notamia bursaria, and other marine Polyzoa.t
(5.) Organs of the Life of Relation.
I have succeeded in making out a distinct nervous system in all the genera with the exception of Urnatella and Pectinatella, which I have had no opportunity of examining, and of Paludicella, in which I have not as yet been able to effect any satisfactory demonstration of its existence. In the phylactolæmatous species, there may be seen attached to the external surface of the esophagus, on its rectal aspect just below the mouth, an oval body of a yellowish colour, and presenting a somewhat lobed outline (Pl. II, fig. 24; V, fig. 5; IX, fig. 7, w). That it is a nervous ganglion there cannot be any doubt, and I have succeeded in distinctly tracing nervous filaments in connection with it. In Cristatella, Lophopus, and other genera with crescentic lophophores, the ganglion may be seen giving off from each side a rather thick chord (Pl. II, fig. 24, x) which immediately enters the tubular arms of the lophophore, and then, after giving off a branch which runs along the root of the lophophore towards the hæmal side, and which sends in its passage a filament to each tentacle on this side of the lophophore, it continues its course (x') along the roof of the arms to their extremity, sending off at regular intervals a filament to each tentacle upon the outer margin of the arm. When it arrives at the extremity of the arm it turns on itself, and in its retrograde course gives off similar filaments to the tentacula placed upon the inner margin. I have thus traced it back to the base of the arms, but have here failed in my attempts to follow it further; it is, however, highly probable that it passes across the lophophore to unite with the corresponding chord of the opposite side. The tentacular filaments are directed towards the intervals between the tentacula. The ganglion also sends off a filament (y), which dives into the substance of the csophagus just behind the mouth ; it is probably distributed to this tube, and to the mouth and epistome, but I have not succeeded in detecting anything like a nervous collar surrounding the æsophagus at this place. There is no other ganglion than the one just described ; and, unless it be the epistome, and possibly in Pedicellina a peculiar ciliated organs in the neighbourhood of the ganglion, nothing which can with any real probability be referred to an organ of special sense has as yet presented itself in any polyzoon.
* Milne-Edwards, Recherches Anatomiques, Physiologiques et Zoologiques sur les Eschares, Ann. des Sci. Nat.,' 2de série, t. vi.
† Busk in 'Transactions of the Microscopical Society of London,' vol. ii. I See note, p. 19.
B. Organs for the Preservation of the Species and the Anatomy and Development of the Bud.
(1.) Ovary and Testis.
True sexual organs have now been satisfactorily demonstrated in several of the genera of Polyzoa, both fresh-water and marine. In Alcyonella and Paludicella, I have succeeded in making a careful examination of the generative system. In each of these both ovary and testis are found in the same cell.* During the months of July and August there may frequently be seen in the interior of the cell of Alcyonella fungosa, a roundish mass (Pl. III, fig. 7, 4) attached by a short peduncle to the endocyst at a little distance within the orifice, and corresponding exactly in position to an ordinary bud, with the early stage of which it may indeed be readily confounded. This body is the ovary. It is filled with spherical ova in various stages of maturity. The testicle, which will be found at the same time and in the same cell with the ovary, is developed in the form of an irregular roundish mass (x), upon a peculiar appendage which is present in all the fresh-water polyzoa I have had an opportunity of examining, and which is always in the form of a long cylindrical, flexible cord, attached by one end to the fundus of the stomach, and by the other to the endocyst near the bottom of the cell. We may, with Huxley, designate this appendage by the term funiculus. The testicle is composed of a mass of spherical cells, each of which contains within it numerous secondary cells, “vesicles of evolution.” (Pl. XI, fig. 17). The visible contents of the vesicles of evolution consist, at first, of nothing more than a well-defined spherical nucleus, and this is subsequently transformed into a spermatozoal filament, which finally escapes by the rupture of the containing cells (figs. 17—23). The spermatozoal filaments, in this genus, are simple vibrioid bodies (fig. 23) without any terminal enlargement. They present distinct though somewhat sluggish undulatory motions. The distal portion of the testis is more developed than the portion which lies nearer to the stomach of the polypide, and the former portion may generally be seen with the undulating spermatozoa projecting from it on all sides, in the form of a dense villosity (Pl. III, fig. 7, X), while some of these, already become free (F), may be seen carried about in the fluid of the perigastric space, and thus brought in contact with the ovary.
In Paludicella, the ovary occupies the same position as in Alcyonella, forming an irregularly shaped body (Pl. X, figs. 3, 4, 4), adherent to the inner surface of the endocyst towards the anterior part of the cell. About the end of June, when I discovered this organ, it was loaded with ova of various sizes, some so small as to require for their detection considerable magnifying powers, while others were almost visible to the naked eye, and seemed ready to burst the
* Van Beneden at one time maintained the unisexualism of Alcyonella, believing that the testis and ovary always occupy separate cells (Quelques observations sur les Polypes d'eau douce, 'Bull. de l'Acad. Roy. de Bruxelles,' 1839). In a subsequent memoir (Dumortier and Van Beneden, Hist. Nat. des Pol. comp. d'eau douce, “Mém. de l'Acad. Roy. de Bruxelles,' tome xvi, Complément) he modifies this view, and comparing the polyzoon in question to plants belonging to the class Polygamia of the Linnean system, he believes that among the different zooids of the same colony, there are some in which the sexes are distinct, and others in which they are united. My own observations, however, are opposed to both these views, and it seems to me evident that the eminent professor of Louvain has not seen the true ovary at all.
restraining membrane of the ovary, and escape into the cavity of the endocyst. Attached by one extremity to the external surface of the stomach, near the commencement of the intestine, and by the other attached to the walls of the cell, and apparently also in connection at this place with the ovary, is a cylindrical flexible chord (anterior funiculus) (Pl. X, figs. 3, 4, 6'), which obeys all the motions of the stomach. It exactly resembles that already described as attached to the fundus of the stomach and bottom of the cell in Alcyonella.
The testicle in Paludicella is an irregularly lobed mass (Pl. X, figs. 3, 4, X), attached, like the ovary, to the inner surface of the endocyst. It is situated near the bottom of the cell, and is thus, as in Alcyonella, separated, by a considerable interval, from the ovary; it is connected with the stomach by a cylindrical chord, or posterior funiculus (Pl. X, figs. 3, 4, 6), similar in all respects to the funiculus of Alcyonella, and, except in position, to the anterior funiculus of the present genus. The testicle was coexistent with the ovary, and was loaded with spermatozoa, multitudes of which projected from its surface, presenting quite the same appearance as in Alcyonella, while many had escaped from the testicle, and were observed to be carried along in the currents of the perigastric fluid, or might be seen clustering round the ovary. The testicle is here, as in Alcyonella, composed of mother-cells (Pl. XI, fig. 24), containing distinctly nucleated vesicles of evolution. The spermatozoa are formed by the transformation of the nucleus. They have a terminal enlargement of an elongated piriform shape (Pl. XI, fig. 25), and exhibit a constant sinuous or undulatory motion.
(2.) Embryology and Gemmation.
Development of the Ovum.—I have succeeded in tracing the development of the ovum through most of its stages in Alcyonella fungosa.
In this polyzoon the mature ovum consists of a granular vitellus, surrounded by a very evident vitellary membrane, on whose internal surface the contents appear frequently to be aggregated in a coarser granular layer (Pl. XI, figs. 26, 27). It presents a large germinal vesicle, and a very distinct germinal spot. After a time the germinal vesicle and germinal spot disappear, and the vitellus undergoes segmentation, and after the mulberry-like condition thus induced has in its turn vanished, we find that the contents of the egg have assumed the form of a roundish or oval body (Pl. XI, fig. 29), richly ciliated on its surface, and provided with a large central cavity, which as yet does not open externally. When liberated from the outer membrane of the ovum, which still confines it, it swims actively through the surrounding water by the aid of the cilia with which it is invested.
As development proceeds, we find the ciliated embryo, while still confined within the coverings of the egg, presenting in some part of its surface an opening which leads into the central cavity; and through this opening an unciliated, hernia-like sac is capable of being protruded by a process of evagination. The unciliated protrusible portion would seem to have been derived by a separation from the walls of the central cavity, and appears therefore to originate by a process of unlining, a true chorization.
The following four diagrams will convey a clearer idea than mere description of the probable stages of this process:
2. Diagrams representing the chorization process in the development of the embryo of Alcyonella. The stage
represented in 3 has not been actually seen, but seems to be the only one that will supply the missing link.
Towards the opening which leads from without into the central cavity the chorization is incomplete, the membrane as it separates being here still held to the walls of the cavity by irregular transverse bands; these bands check the entire evagination of the membrane, but after a time they disappear, and then the unlining and evagination are perfect. In the interior of the protrusible portion, and before the disappearance of the transverse bands, a polypide is developed (Pl. XI, fig. 30). This appears to take place in a manner quite similar to that by which new polypides are produced by gemmation from the walls of the endocystal cavity in the adult. The gemmation of the first polypide is immediately followed by that of another close beside it (Pl. XI, fig. 31), so that the young polyzoon has now the appearance of a transparent, closed sac, filled with fluid, the posterior part ciliated, the anterior destitute of cilia and partially or entirely pushed back into the posterior by a process of invagination ; while the sac carries within it two young polypides, which are suspended from the inner surface of the unciliated portion. The arms of the lophophore in the young polypides are at first but slightly developed, and there is as yet no trace of tentacula (Pl. XI, fig. 30“); these soon present themselves in the form of minute tubercles, at first confined to the body of the lophophore (Pl. XI, fig. 30'), and then extending along the arms, which at the same time acquire increased proportional length ; the tentacula gradually elongate themselves and acquire cilia.
At the same time, the alimentary canal, represented at first by a single small cavity, hollowed out in the body of the polypide, is undergoing development, and oesophagus, stomach, and intestine begin to be distinguishable. The great retractor muscles have become evident, and the funiculus may be seen extending from the base of the polypide to the walls of the sac (Pl. XI, fig. 31). The polypides have, at first, no communication with the exterior, but at an early stage the tentacular sheath, with the parieto-vaginal bands, had become evident, and the fluid in which the embryo floats within the walls of the ovum is soon afterwards admitted to the lophophore of the young polypides. The parieto-vaginal bands would seem to be drawn out by a process of separation from the walls similar to that just described.
The embryo is still contained within the external membrane of the ovum, which, however, has become much distended, in accordance with the increasing size of the included parts, and at length, giving way, allows the embryo to escape.
The free embryo (Pl. XI, figs. 32, 33, 334, 33') now swims actively through the surrounding
fluid of the perigastric space. It sometimes remains stationary, with the unciliated portion, in which the polypides are suspended, protruded from the ciliated (Pl. XI, fig. 32); but most frequently this unciliated portion is withdrawn within the ciliated, which is then closed over the aperture (Pl. XI, fig. 33). In this condition the little animal assumes a piriform figure, with the small end corresponding to the aperture. It is thus carried through the surrounding fluid by the vibration of its cilia, performing rapid and elegant motions, always swimming with the broad end foremost, and at the same time revolving gracefully on its axis.
The complete evagination of the unciliated portion is still prevented by the bands already mentioned, but we now find that these bands, which must not be confounded with the permanent parieto-vaginal bands of the adult, have disappeared, and the evagination has become complete (Pl. XI, fig. 34). The unciliated portion is now no longer capable of being withdrawn within the ciliated, with which it has become directly continuous, while the cilia themselves disappear from the ciliated portion, and the entire sac becomes enveloped in an ectocyst, to constitute the cell of the adult polyzoon. The subsequent changes are produced by the gemmation of new polypides, with their proper ectocysts and endocysts.
Plumatella fruticosa presents similar developmental phenomena; the ciliated larva, however, in this species, differs from that just described, in having its polypide single (Pl. XI, fig. 35).
If a specimen of Alcyonella fungosa be cut into small pieces, under water, in the month of July or August, the ciliated embryos will be liberated in abundance, and may be examined with facility.
Gemmation.—The development of gemmæ has already been partially traced in the description just given of the larva of Alcyonella ; we must now follow it, however, a little more in detail, as it is presented by the buds formed in the adult polyzoon.
With the exception of some peculiar forms of gemmæ, to be presently described, these bodies always originate in the endocyst. In Lophopus, Alcyonella, Plumatella, and Fredericella, they occur without any very regular order near the orifice of the cell. In Cristatella the gemmæ are produced very regularly from constant points on the sides of the previously existing cells, and the new cells thus produced remaining in apposition with one another, and never becoming extended into branches, constitute several concentric series on the surface of an expanded disc. In Paludicella they also arise with much regularity from fixed points a little below and at each side of the orifice of the previously formed cells; and here, not continuing in apposition, the new cells form branches, which, from the fixed points at which they originate, and the constant angle at which they are given off from the parent-cell, confer upon the whole colony a greater regularity than is met with in the other branched forms of the freshwater Polyzoa.
Most of the steps in the development of the gemma may be traced with considerable facility in Paludicella. In the earliest condition in which I have been able to observe it, the gemma appears here as a minute tubercle, projecting from the external walls of the cell, and filled with a granular parenchyma (Pl. XI, fig. 1). It now becomes elongated (fig. 2), and we soon find it noilowed out into a cavity, which communicates with the interior of the parentcell. The tubercle, with its cavity, increase in size, and the gemma (fig. 3) is now found to consist of an external envelope, continuous with the ectocyst of the parent-cell, and of a thick,