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which Dr. A. Farre” attributes so large a share in the production of this act among the marine Polyzoa, can at most raise the lophophore and tentacula a very short distance, and can exercise no exsertile influence on the inferior portion of the polypide, which, indeed, it must rather tend to repel into the bottom of the cell; while in all the fresh-water genera, with the exception of Paludicella, the oesophagus, in the retracted state of the polypide, is scarcely at all bent, so that here its agency in exsertion is at once out of the question. Let us suppose the polypide withdrawn into the recesses of the cell, and that hunger or some other stimulus impresses on it a desire of protrusion. The endocyst now contracts on the perigastric fluid, which, pressing on the polypide, forces it onwards towards the orifice; at the same time the vaginal sphincter relaxing, affords to the cone of tentacula a free passage through the tube of the inverted endocyst. The succeeding steps in the process take place somewhat differently in the two great groups. In Plumatella and the other fresh-water phylactolaematous genera, as the polypide continues to advance from the cell, the invaginated endocyst is gradually carried out with it by a process of evagination, which proceeds up to a certain point, where it is stopped by the action of the posterior parieto-vaginal muscles, which, by straining upon the invaginated membrane, had already afforded a fixed line, on which it rolled outwards during eversion, This line constitutes the extreme limit of eversion, and that portion of the invaginated endocyst which lies between it and the mouth of the cell remains permanently invaginated. In Paludicella the process is somewhat more complicated; here the relaxation of the anterior parieto-vaginal muscles permits the eversion of the endocyst, but only to a limited extent, for the posterior fibres of these muscles soon check its further progress, keeping one portion permanently invaginated, and affording a fixed point on which the remainder may roll outwards. This second portion of the invaginated membrane, which in the retracted state constitutes the tentacular sheath, continues to be carried outwards by the advancing polypide, the posterior parieto-vaginal muscles slowly relaxing to admit of it. These muscles, however, after a certain time refuse to suffer further relaxation, and thus afford a second check to the evagination of the membrane. Thus we have two small permanent invaginations existing after the completion of the protrusive act (Pl. X, fig. 4). One of these is placed within the other. and gives rise to the membranous cup which projects from the lips of the orifice in the exserted state of the polypide. This cup, therefore, which may plainly be seen under a proper illumination to consist of a membrane doubled into itself, is nothing else than the imperfectly evaginated tentacular sheath. It may be witnessed during the act of protrusion in Plumatella and other genera; but in these it is a mere temporary condition, being obliterated on the completion of the act. When the protrusion of the polypide is completed, the last act in all the species is the display of the tentacula, which had previously been all drawn together into a close cone or cylinder; and scarcely any more pleasing sight can be presented to the microscopic observer than the spreading out of the beautiful crown and the excitement of the vortices in the surrounding fluid, by the countless cilia which instantly commence their untiring vibration on the sides of the tentacula. The mechanism of retraction is easily understood. Here the perigastric fluid being no
* Observations on the Minute Structure of some of the higher forms of Polypi, ‘Philosophical Transactions,’ 1837.
longer pressed upon by the contraction of the endocyst, the great retractor muscles act directly on the polypide and withdraw it into the cell, the anterior and posterior parietovaginal muscles in Paludicella drawing after it as it descends that portion of the endocyst which had been carried out during protrusion; in the other genera, however, the anterior muscles would seem to take no part in this act. When the retraction is complete, the sphincter closes the tentacular sheath, and the polypide rests secure in the recesses of the cell. The muscles of these animals are especially interesting in a physiological point of view, for they seem to present us with an example of true muscular tissue reduced to its simplest and essential form. A muscle may, indeed, here be viewed as a beautiful dissection far surpassing the most refined preparation of the dissecting needle, for it is composed of a bundle of elementary fibres, totally separate from one another through their entire course. The fibres of the great retractor muscle are distinctly marked with transverse striae, a condition, however, which is not at all times equally perceptible; and some of our best observers have denied to the Polyzoa the existence of striated fibre. I have, however, by repeated observations, satisfied myself of the striated condition of the fibre in the great retractor muscle in the fresh-water genera (Pl. IX, fig. 6). In Paludicella, I have seen this state beautifully marked through the pellucid cell in the whole extent of the retractor muscle while the fibres were on the stretch in the exserted condition of the polypide; and in all the other genera which I have had an opportunity of examining it has, under favorable circumstances of observation, been more or less visible. In order to witness it in perfection, the fibre must be on the stretch ; for when this is torn from its attachments or lies relaxed on the bottom of the cell, the striae become very obscure. When the broken extremity of a fibre is examined, the fracture will be found to have occurred in a plane perpendicular to the axis of the fibre, never presenting an uneven or lacerated surface, and a marked tendency to separate into discs may be recognised in the detached and broken fibre. Indications of a very delicate investing sarcolema may also be occasionally witnessed. When the fibre is in an uncontracted state, it would seem to be perfectly cylindrical ; and the normal act of contraction is so momentary that its condition during this act cannot be witnessed. When, however, the living polypide is torn from its cell, the ruptured fibres which continue attached to its body are thrown into a state of spasmodic contraction, and then it will be seen that they lose their cylindricity and become irregularly swollen at intervals, while the whole fibre has much increased in thickness: in this state we may also observe it obscurely striated. The swellings here visible in the contracted fibre are quite different from the peculiar knots described by Dr. A. Farre, in the muscles of the marine Polyzoa. Such knots do not exist in the fresh-water species—at least I have never seen them—with the exception, perhaps, of certain little swellings, which may be occasionally witnessed in the parietal muscles of Paludicella and in the superior parieto-vaginal muscles of Plumatella. In Paludicella I have witnessed a curious phenomenon presented by the muscular fibre. In this polyzoon the fibres of the great retractor muscle, while lying relaxed in the bottom of the cell after the retraction of the polypide, may frequently be seen to present a singular motion, impressing you with the idea of a cluster of writhing worms. It is only in the great retractor muscles that I have succeeded in detecting the striated condition of the fibre. It has been already shown (p. 12) that the fibres occurring in the endocyst (parietal - 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 phylactolaematous species, there may be seen attached to the external surface of the oesophagus, 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, w) 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 haemal side, and which sends in its passage a filament to each tentacle on this side of the lophophore, it continues its course (a) 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 oesophagus 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 oesophagus 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 organ; 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.
f 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 fun/osa, a roundish mass (Pl. III, fig. 7, p) 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 (Š), 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, b), 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 funyosa.
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.