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(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.) Vaginal 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 striæ, 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
* Loc. cit.
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.
(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 hæmal, 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.) Vaginal 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 œsophagus, to
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 œsophagus, 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 phylactolæmatous 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 striæ, 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 striæ 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