« AnteriorContinuar »
structure ; it resists putrefaction longer than the external cellular layer, and forms the immediate walls of the tubular cavity. Nervous filaments, and certain bands, probably muscular, to be presently described, may be traced as far as the root of each tentacle, and doubtless also enter into its structure. In Cristatella, a minute cavity, which looks as if it were cut off from the rest of the tube, may be very plainly seen in the extremity of each tentacle; this condition would also seem to exist in other genera, but it is nowhere so well marked as in Cristatella.
In all the fresh-water genera, with the exception of Paludicella, and possibly of Urnatella, the entire plume of tentacula is surrounded at its base by an exceedingly delicate
water Phylactolæmata is probably united. In order to understand this relation, it is necessary to conceive of the polypide as partially retracted; the invaginated portion of the endocyst must then be viewed as adherent to the external portion externally, and to the tentacula (and calyx ?) internally. From this arrangement it is obvious, that the exsertion and retraction of the polypide must be
When the polypide desires to withdraw under cover of the cell, the free portion of each tentacle is rolled inwards as far as the margin of the calyx-like cup, and then the mouth of the
up is closed over the whole by the action of a well-developed sphincter muscle, the tentacular crown being at the same time slightly drawn backwards by some retractor fibres, which may be seen extending from the upper part of the pharynx to the base of the cell. The sphincter would seem to represent a condensed band of the parietal muscles of other Polyzoa, or it is probably homologous with the vaginal sphincter, which is nothing more than a peculiar development of these muscles in the invaginated endocyst. The retractor fibres are obviously homologous with the great retractor muscles of the others.
The mouth, which occupies its normal position in the body of the lophophore, opens into au @sophagus, which after a short course terminates in the stomach. This is a large sac which lies in the bottom of the cell ; close to the pyloric orifice it becomes much constricted, and this constricted portion passes into the intestine whose origin is nearly on a level with the cardiac orifice, but separated froin it by a considerable space. The intestine passes first forwards and towards the neural side, then turns towards the mouth to open after a wide dilation between the arms of the lophophore in the bottom of the tentacular crater.
The concave margin of the body of the lophophore is raised into a kind of thickened rim, which arches over the mouth, so as to constitute an epistome thickly ciliated on its oral aspect, but not provided with muscles like the epistome of the ordinary phylactolæmatous Polyzoa.
The hepatic layer, which, in other Polyzoa, forms a continuous lining extending over nearly the whole of the internal surface of the stomach, is here confined to the anterior portion of this sac, in the space between the cardiac and pyloric orifices, where it constitutes a dark brown glandular mass composed of distinct cells, which are filled with the characteristic brown secretion. The rest of the stomach, as well as the oesophagus, is lined with vibratile cilia, which are especially developed in the vicinity of the pylorus.
The ovary is an irregularly shaped mass, situated between the cardiac and pyloric orifices of the stomach, and the animal appears to be unisexual, the testis occupying the place of the ovary in other individuals.
The stalk on which the cell containing the polypide is elevated, constitutes a very characteristic feature of Pedicellina. It is a tube in whose walls both ectocyst and endocyst can be demonstrated ; it contains straight muscular fibres, which extend from the base of the cell to the point of attachment of the stalk; and besides these, more delicate circular fibres can also be detected in it. By the action of the straight and circular fibres, various motions, especially those of flexion and extension, can be given to the stalk, and these motions, when witnessed in a living and active group of Pedicellina, present an appearance in the highest degree novel and interesting.
transparent membrane in the form of a cup or calya (Pl. II, fig. 24; V, fig. 5; IX, fig. 7, m).
is adherent to the back of the tentacula, and its margin is in most instances prolonged more or less upon each tentacle, as a narrow triangular process, so as to present a sort of scalloped or festooned appearance; the festooning of the margin is most marked in Fredericella ; in some species of Plumatella it is scarcely perceptible. A high power of the microscope, and carefully adjusted illumination, will enable us to detect in the calyciform membrane certain delicate anastomosing lines. These appear to indicate the surfaces of contact of cells of which the membrane would thus seem to be composed; they are particularly evident in Cristatella. It is curious enough that the calyx should be exactly coincident with the presence of an epistome in all the fresh-water Polyzoa; unless Urnatella should prove to be an exception, which is not likely, though we are not yet sufficiently acquainted with the structure of the genus to include it in this generalization. In no marine polyzoon has a calyx yet been detected, unless we admit the by no means improbable supposition that it enters partly into the composition of the calyx-like cup which surrounds the base of the tentacles in Pedicellina, the only marine genus in which an epistome is also represented.*
The perigastric space, and interior of the tentacula and lophophore, all freely communicate with one another, and are filled with a clear fluid, in which float numerous particles of very irregular form and size. In this fluid may be observed a constant rotatory motion, rendered apparent by the floating corpuscles as they are whirled away under the influence of the currents. That the fluid thus contained in the perigastric space, and thence admitted into the tentacula, consists mainly of water which had obtained entrance from without, there can, I think, be little doubt, and yet I have in vain sought for any opening through which the external fluid can gain admittance to the interior. I have allowed the transparent genera Cristatella and Lophopus to remain many hours in carmine without being able to detect a single particle of this pigment in the perigastric space, though I have seen this space rapidly empty itself on the removal of the animal from the water, and again fill on restoring it to its natural element. Van Benedenf believed that he had detected in Alcyonella apertures, which he names “ bouches aquifères," at the base of the tentacula ; but this distinguished naturalist is certainly here in error, as indeed he himself subsequently admits. Meyen asserts the existence of an aperture in the vicinity of the anus, through which, he tells us, he has witnessed the escape of an egg in Alcyonella ;* and Siebold admits the correctness of this statement, and considers the aperture described by Meyen to be that through which the external water is admitted to the interior. I have, however, fully convinced myself that no such aperture exists, and the phenomenon described by Meyen must certainly be due to an accidental rupture of the tissues, though Van Beneden describes the passage of the eggs through an aperture similarly placed in the marine genus Laguncula. I It is possible that certain apertures may exist in some of the tissues of the animal so minute as to defy our attempts at detection, and yet capable of permitting a passage of Auid ; some facts already recorded (page 13), would seem to point to the existence of a system of tubes in the substance of the endocyst, which may afford the necessary channels of communication, or it may be that it is in simple transudation through the walls of the alimentary canal that we are to seek for the true mode in which the external water passes into the perigastric space.
The stalk of Pedicellina must be viewed as homologons with the posterior part of the cell in the unstalked forms of Polyzoa. It is simply this portion of the cell become so much constricted as to be no longer capable of containing the polypide, which is in consequence pushed onwards into the wider portion which now constitutes the proper cell. The muscles of the stalk have their representative in the muscles developed in the walls of the cell of Polyzoa.
Between the esophagus and rectum, but separated from the latter by the whole mass of the generative system, is the nervous ganglion, close to which, and lying between it and the oesophagus, is a peculiar organ in the form of a minute tubular cavity clothed with actively vibrating cilia. I was unable to follow this organ through its whole extent, or determine its exact relations, as it appeared to lose itself beneath the opacity of the surrounding structures. Its situation would suggest the probability of its being an organ of sense. It is possible, however, that it is a portion of a more extensive system of tubes, a supposition which some appearances seem to warrant, and then we might perhaps view it as indicating the presence of a water-vascular system.
* See previous note. + Quelques Observations sur les Polypes d'eau douce, 'Bull. de l'Acad. Roy. de Bruxelles,' 1839.
The real signification of the perigastric fluid is a point whose determination must be of great importance in the physiology of the Polyzoa. As has just been said, it is by no means homogeneous, and numerous corpuscles of very various and irregular shape may be observed to float through it and be carried about by its current. Some of these corpuscles are, doubtless, spermatozoa ; others are of no definite shape, and look like minute portions of the tissues separated by laceration.
If it be admitted, as I think it must be, that the perigastric fluid consists mainly of water which has obtained entrance from without, it then corresponds to a true aquiferous system subservient to a respiratory function. But it also without doubt receives certain products of digestion which had transuded through the walls of the alimentary canal; it thus connects itself with the digestive system. It is, moreover, the only representative in these animals of a sanguiferous circulation, for in the Polyzoa there is certainly no trace of a heart, nor can anything referable to a true vascular system be detected. The perigastric circulation, therefore, unites in itself the triple function of a chyliferous, sanguiferous and respiratory system. And the fluid in question would correspond to the “chyle-aqueous fluid,” which plays so important a part in the economy of the lower animals, and whose nature has been recently well elucidated by the researches of Dr. Williams.
The next point of interest to determine, with regard to the perigastric fluid, is the cause of the peculiar currents observed in it. These currents, which extend into the tentacular crown, were long ago observed by Trembley|| in Lophopus crystallinus ; but this author contented himself with simply recording their existence, and made no attempt to explain them. Nordmann, who observed them both in fresh water and marine genera, not being able to detect any trace of cilia or other moving power, compared them to the currents in the cells of Chara. That they are produced partly by the action of vibratile cilia, and partly by the muscular
Meyen, Naturgeschichte der Polypen, Isis, 1828.
+ Loc. cit. § 41. # Recherches sur l’Org. des Laguncula, 'Nouv. Mém. de l'Ac. Brux.,' xviii. Ø On the Blood proper and Chyle-aqueous Fluid. · Phil. Trans. 1852. || Mém. pour l'Hist, des Polypes d'eau douce.
Micrographische Beiträge, Bd. ii. p. 75.
contraction of the endocyst, there can, however, now be no doubt. Van Beneden* tells us that he has seen these cilia, not only on the walls of the perigastric space, but on the external surface of the alimentary canal. I cannot, however, confirm their existence in the latter situation ; indeed, my own observations are entirely opposed to their presence on the alimentary canal; and I cannot help thinking that this statement of Van Beneden is connected with some error of observation. I have, however, most distinctly seen them on the inner surface of the upper part of the tentacular sheath in certain species during the exserted state of the polypide (Pl. IV, fig. 4); on other parts of the endocyst I have not succeeded in detecting them by direct observation ; but the peculiar acceleration which the motion of the circulating corpuscles experiences when these approach the walls of the perigastric space, plainly indicate the presence of vibratile cilia in this situation.
(4.) Muscular System.
The muscular system is highly developed ; we shall first consider it in the phylactolæma. tous Polyzoa, and afterwards attend to its disposition in Paludicella.
In all these the disposition of the muscles is exceedingly similar ; eight distinct sets may be considered as more or less positively demonstrated.
(1.) Retractor Muscles of the Polypide.
These, which are the largest and most powerful muscles of the animal, consist of two fasciculi (Pl. IV, fig. 4; V, fig. 5; IX, fig. 7, n), which arise far back from the inner surface of the endocyst, and thence pass forwards, one along each side of the alimentary tract, to be inserted into the upper part and sides of the @sophagus. A few accessory fasciculi (Pl. V, fig. 5, 0) may also be generally seen arising near the origin of the former, and inserted into the sides of the stomach. The action of the retractor muscles is very obvious; having a comparatively fixed point of attachment near the bottom of the cell, they retract the whole alimentary canal with the tentacular crown, so as to place them in a state of security in the interior of the cænæcium.
* Quelques Observations sur les Polypes d'eau douce, loc. cit.
(2.) Rotatory Muscles of the Crown.
These also consist of two fasciculi (Pl. IV, fig. 4; V, fig. 5; IX, fig. 7, p), which arise along with the set just described, and passing forward in company with these, separate from them at some distance below the crown, and thence pass outwards to the right and left, to be inserted each into its own side of the lophophore. Action : they rotate the tentacular crown, and depress the lobes.
(3.) Tentacular Muscles?
Under this name may perhaps be described a set of delicate parallel bands (Pl. V, fig. 5,9). to which attention was first directed by Van Beneden, and which may be observed running from below upwards upon the margin of the lophophore; these bands are continuou with one another below, and when they arrive at the intervals between the roots of the tentacula, each divides into two others, which would appear to run along the opposed sides of two neighbouring tentacula. M. Van Beneden considers them as muscles destined to act on the tentacula; but it must be admitted that we are scarcely justified in pronouncing decidedly on the muscular nature of these bands, which certainly do not present any distinctly fibrous structure. If they be truly muscular, it is to their action that the various motions observed in the tentacles would seem to be chiefly due. The margin of the lophophore in the interval of the bands presents an oval transparent space, which looks exactly like an aperture, and it would seem to be these spaces which M. Van Beneden has taken for “ aquiferous mouths;" after very careful examination, however, I have convinced myself that no aperture exists here, the apparent mouths being merely transparent spaces in the lophophore.*
(4.) Elevator Muscle of the Epistome.
This is a small but very evident fasciculus (Pl. II, fig. 24, r), occupying the interior of the epistome, and visible through its transparent walls; it arises from the lophophore within the cavity of the epistome, and then passing obliquely across the cavity, is inserted into the inner surface of the oral wall of the epistome. Action : it elevates the epistome, and draws it away from the mouth.
* M. Van Beneden has himself given up his earlier views upon this point, and has referred the appearance in question to its proper cause. Dumortier and Van Beneden, Hist. Nat. des Polypes composés d'eau douce, 2de partie, ‘Mém. de l'Acad. Roy. des Sc. et Belles lettres de Bruxelles, Compl., t. xvi.