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nutritious elements, portions in the form of roundish pellets become separated at intervals from the mass, and are slowly propelled along the tube towards the anus, where, having arrived, they are suddenly ejected into the surrounding water and rapidly whirled away by the tentacular currents. It was these excrementitious pellets that Turpin mistook for unarmed ova in Cristatella.
(3.) Organs of Respiration and Circulation.
Upon the tentacular crown and the walls of the perigastric space would seem, among the Polyzoa, chiefly to devolve the function of bringing under the influence of the aërating medium the nutritious fluid of their tissues.
The tentacular crown of a Polyzoon consists of two portions, namely, first, a sort of stage or disc (the lophophore) (Pl. II, fig. 24; IX, fig. 7; X, figs. 3, 4, Å) which surrounds the mouth ; and secondly, a series of tentacula () which are borne upon the margin of the lophophore. The lophophore is throughout almost the entire class of an orbicular figure; but in the fresh-water genera Cristatella, Lophopus, Plumatella, and Alcyonella, its neural margin, or that which corresponds to the side of the rectum, is extended into two long triangular lobes or arms, so as to cause the lophophore in these genera to present the form of a deep crescent, round whose entire margin the tentacula are borne in one continuous series. This condition of the lophophore is found in no marine species. In Fredericella the arms of the crescent are obsolete, and the lophophore here may, on a superficial view, appear orbicular; but a careful examination will render manifest its departure from the orbicular form, the side corresponding to the arms of the crescent being slightly prolonged, while the bilaterality is rendered still more decided by the presence of an epistome. Paludicella is the only fresh-water genus, if we except Urnatella, not yet sufficiently examined, in which not the slightest trace of bilaterality can be detected (Pl. X, fig. 5). With the exception of Pedicellina, which presents a condition of the lophophore altogether unique and exceptional,” and some other marine genera in which a slight tendency to assume a bilateral form may be observed, but which never possess an epistome, the lophophore in the marine genera is always orbicular. The lophophore in all the genera forms the roof of the perigastric space; in the species with crescentic lophophores the arms of the crescent are tubular and open into this space; the interior of the arms is clothed with vibratile cilia.
* The structure of Pedicellina has been hitherto entirely misunderstood. I have had an opportunity of carefully examining this Polyzoon, and as it is constructed on a plan quite peculiar, and may tend to throw light on the morphology of other types, it may not be out of place to give a description of it here.
The general appearance of Pedicellina is that of a gymnolaematous Polyzoon with its nearly spherical cell seated on the extremity of a long peduncle. The gymnolaematous character, however, is wholly deceptive, and depends on a remarkable condition of the lophophore, which, though at first sight orbicular, is truly bilateral and phylactolaematous, but nevertheless constructed on a type peculiar to itself, and of which no other Polyzoon affords any example, unless further examination shall show a
similar disposition in Urnatella. The two arms of the lophophore have the tentacula confined to their outer margin. They are
The tentacula are tubular, closed at their free extremity, and opening by the opposite through the lophophore into the perigastric space; in all the Polyzoa they are armed upon their opposed sides with vibratile cilia, arranged in a single series, and vibrating towards the remote extremity of the tentacle upon one side, and towards the base on the other. Two very distinct layers (Pl. IX, fig. 5) enter into the structure of the tentacula. The external layer consists of rounded cells filled with a colourless fluid, and often presenting a bright nucleus. Some of those cells which lie upon the back of the tentacle become in certain genera enlarged, giving a vesicular appearance to the organ; this is particularly evident in Cristatella. The internal layer is a delicate transparent membrane, in which I could detect no trace of
approximated at their extremities, and thus, instead of constituting an open crescent, they form a ring enclosing a space which embraces within it the termination of the intestine.
Fig. 3. Fig. 4.
Ectocyst. b. Endocyst. c. Invaginated portion of endocyst adherent to the bases of the tentacula and entering into the formation of the cup. d. Muscular fibres surrounding the margin of the cup. e. Mouth.
f. (Esophagus. g. Stomach. h. Rectum. i. Anus. k. Epistome. l. Ganglion. m. Generative organ. n. Retractor muscle. o. Lophophore.
The tentacula are connected to one another at their base by a membrane which adheres to their back, extending forwards for about one third of their length, and constituting a cup. This membrane passes uninterruptedly from the extremity of one arm of the lophophore to that of the other, thus binding together the two tentacula which spring from the points of the arms, and completing the tentacular crater so as entirely to disguise the hippocrepian character, and give to the crown of tentacula the form presented by the infundibulate genera. The result of this arrangement is, that the anus presents the anomalous condition of opening within the tentacular crater. It will be easily seen, however, that the position of the anus though within the circle of tentacula is still properly external to the lophophore, and thus really occupies its normal position in the concavity of the crescent.
The membranous cup which surrounds the base of the tentacular crown is not homologous with the calyx of the ordinary hippocrepian Polyzoa, but would seem rather to represent a permanently invaginated portion of the endocyst, with which, however, a true calyx equivalent to that of the fresh
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 Phylactolaemata 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 very limited. 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 cup 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 an opsophagus, 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 from 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 phylactolaematous 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 transparent membrane in the form of a cup or calya (Pl. II, fig. 24; V, fig. 5; IX, fig. 7, m). This cup 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 aquiferes,” at the base of the tentacula; but this distinguished naturalist is certainly
given to the stalk, and these motions, when witnessed in a living and active group of Pedicellinae, present an appearance in the highest degree novel and interesting. The stalk of Pedicellina must be viewed as homologous 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 oesophagus 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. 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.t 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. 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 fluid ; 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 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