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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 phylactolaema. 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 oesophagus. 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 coenoecium.
* 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 2
Under this name may perhaps be described a set of delicate parallel bands (Pl. V, fig. 5, 4). 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 continuous 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.
(5.) Anterior Parieto-vaginal Muscles.
These consist of numerous short bands (Pl. IV, fig. 4; V, figs. 5, 6; IX, fig. 7, 8), which arise all round from the inner surface of the endocyst, commencing close to the line of invagination, and extending for some distance backwards. From this origin they pass transversely inwards, and are inserted into the opposed surface of the invaginated endocyst. Their action will dilate the invaginated endocyst, and assist in keeping it permanently inverted.
(6.) Posterior Parieto-vaginal Muscles.
These consist of several radiating bands (Pl. IV, fig. 4; V, figs. 5, 6 ; IX, fig. 7, t), longer and stronger than the last, behind which they arise, from the inner surface of the endocyst in a single plain perpendicular to the axis of the cell, and thence passing upwards and inwards, are inserted into the sheath in a plane parallel to that of their origin, and just behind the termination of the anterior parieto-vaginal muscles. Their action steadies the sheath, and regulates its position during the protrusion of the polypide, while they form a fixed plane on which it may roll outwards with the polypide in the act of protrusion.*
(7.) Waginal Sphincter.
The vaginal sphincter is a circular band (Pl. V, fig. 6, u) surrounding the termination of the invaginated endocyst at the point where the latter passes into the tentacular sheath. Though a contraction of the endocyst at this spot, as if occasioned by the action of a powerful sphincter, may be always observed when the polypide is completely retracted, yet the demonstration of an actual muscle is by no means easy. I have, however, convinced myself of the existence of a distinct structure at the place where the contraction occurs, and, though the presence of fibres is but obscurely indicated, I have no hesitation in viewing this structure as a sphincter muscle on which the contraction in question is dependent. The action of the sphincter closes the sheath after the recession of polypide, and thus protects the latter from all annoyance from without.
* Though I have here described, under the names of superior and inferior parieto-vaginal muscles, the two sets of bands which extend between the walls of the cell and the permanently invaginated portion of the endocyst, I am not by any means without doubts as to their being really muscular, and their action after all may be that of simple ligaments. From the study of development in these Polyzoa, it appears highly probable, as we shall afterwards see, that the invaginated portion of the endocyst is originally separated from the proper walls of the cell by a process of chorization, and while this is taking place the bands in question would seem to be drawn out by the act of separation. There is certainly no reason why muscular fibres may not be developed in them, and the condition of the corresponding parts in Paludicella, as well as in certain marine Polyzoa, is altogether in favour of their muscularity; but they do not always present the appearance of distinct fibrous fascicles, and I have been unable to detect in them the striae, or see them breaking into the discs, which may be observed in the great retractors of the polypide.
(8.) Parietal Muscles.
In the description of the histological structure of the endocyst, muscular fibres (Pl. V, figs. 5, 6, v) were mentioned as entering into the composition of this tunic, and taking a transverse direction round the cell. These may be designated by the name of parietal muscles. They may generally be seen very distinctly towards the anterior extremity of the cell, but it is not always easy to determine how far backwards they extend, as in most cases, the internal structures soon become concealed under the increasing opacity of the superjacent tissues. In the more transparent genus Lophopus, they seem to be present throughout the whole endocyst. By their action they evidently constrict the endocyst in a transverse direction, and thus aid in the protrusion of the polypide. A muscular tissue is also very evident in the walls of the stomach; this has already been described in connection with the histology of the alimentary canal, and need not be further referred to here.
b. Muscles of Paludicella.
The muscular system of Paludicella differs in some important points from that of the species with bilateral lophophores. The muscles may here be divided into five sets:
(1.) Retractor Muscle of the Polypide.
This (Pl. X, figs. 3, 4, n) resembles in attachments and use the corresponding muscle in the other species, but is not so distinctly divided into two separate fasciculi.
(2.) Anterior Parieto-vaginal Muscles.
These constitute four strong fasciculi (Pl. X, figs. 3, 4, 8), which, arising from the sides of the cell near the top, are inserted into the opposed surface of the invaginated endocyst. The fibres of each fasciculus are inserted one after another in a straight line, commencing near the line of invagination, and extending for some distance down the invaginated tunic. These four lines of insertion are placed at nearly equal distances from one another, and thus cause the orifice and invaginated tube to assume a regular quadrilateral figure. The corneous ribs already described correspond to the centre of the intervals between the insertion of the muscles.
Mr. Hancock" enumerates, under the name of superior tube retractors, two small additional fasciculi, which he describes as originating below those just mentioned, and as inserted also below them into the invaginated tube, their insertion becoming of course superior or anterior to them when the tube is evaginated during the exserted state of the polypide. The marine Polyzoa certainly afford an analogy for the existence of these muscles; but, though I have carefully sought for them in Paludicella, I have not succeeded in detecting them here as distinct fasciculi, and I prefer viewing the “superior tube-retractors” of Hancock as some of the posterior fibres of the anterior parieto-vaginal muscles. The action of the anterior parieto-vaginal muscles assists in the invagination of the tube, and dilates it when completely retracted ; they thus act as antagonistic to the vaginal sphincter, while the posterior fibres will check the complete evagination during exsertion.
* Loc. cit.
(3) Posterior Parieto-vaginal Muscles.
These are about four thin fasciculi (Pl. X, figs. 3, 4, t), first pointed out by Mr. Hancock; they arise from the inner surface of the endocyst near the top of the cell, two upon the haemal, and two upon the neural side, and are inserted into the opposed surface of the tentacular sheath. Their action checks the complete evagination of the sheath in the way we shall presently see.
(4.) Waginal Sphincter.
This was also pointed out for the first time by Mr. Hancock. It consists of a set of fibres (Pl. X, fig. 3, u) which run transversely round the invaginated tunic. I have not succeeded in dividing it into an inferior and superior set, as described by Mr. Hancock. Its action closes the invaginated endocyst after the retraction of the polypide.
(5.) Parietal Muscles.
These are numerous, short but strong, and very evident fibres (Pl. X, figs. 3, 4, v), which run transversely in the endocyst in small groups of two or three fibres each, embracing about a third or fourth of the circumference of the cell. Their action compresses the endocyst, and by thus diminishing the cavity of the cell, effects the exsertion of the polypide.
The description now given of the muscular system in the fresh-water Polyzoa, will enable us to understand the mechanism by which the protrusion and retraction of the polypide are effected.
The grand agency to which we must assign the protrusive act, is without doubt the contraction of the endocyst effected by the parietal muscles, or by the general contractility of the tunic itself; and, indeed, it does not seem possible to refer the act of protrusion to any other cause than the consequent pressure of the perigastric fluid against the body of the polypide, and the necessary compulsion of the latter to move in the direction of least resistance, or through the orifice of the cell: for the mere straightening of the Gesophagus, to