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Lophopus, also, at first sight, conveys the impression of being destitute of an ectocyst, and having the place of this tunic supplied by a peculiar unorganized gelatinoid secretion, in which the colony is enveloped (Pl. II, figs. 2, 3). This gelatinous-looking investment is, however, a true ectocyst; it consists of a membranous tunic of great delicacy, apparently enclosing a perfectly transparent and colourless fluid, probably in the meshes of a sort of areolar tissue. I have not, however, succeeded in making out in it any distinct structure, but its membranous nature becomes at once manifest when the animal has undergone partial desiccation, for then the ectocyst is thrown into folds by losing a portion of the fluid which had been imprisoned in it. Neither Trembley nor Baker takes any notice of this gelatinoid envelope. M. Dumortier mentions it, and represents it in his figure,” while M. Van Beneden believes it to be an accidental investment acquired by the animal during confinement.t
The ectocyst in Paludicella is formed of a smooth pergamentaceous semi-transparent membrane, free from earthy deposit, and of a deep brown colour. Towards the orifice of the cell it becomes thin and delicate, and is here strengthened by four longitudinal horny ribs (Pl. X, fig. 3, 0"). The part of the ectocyst to which the ribs are attached is carried inwards during extreme retraction of the polypide. These ribs I look upon as the true homologue of the selae which crown the cell in Bowerbankia and other ctenostomatous Polyzoa ; if these setae were reduced in number to four, and instead of being free were attached along their entire length to the sides of the cell, they would at once be converted into the ribs of Paludicella; the fact of the setae in the ctenostomatous Polyzoa being connected to one another by a delicate membrane does not in the least invalidate the view here taken, and the circumstance of their being detached from the sides of the cell in these Polyzoa will account for the different mode in which they are withdrawn during retraction.
In certain species of fresh-water Polyzoa, transverse septa exist between the cells. They are formed both by the ectocyst and endocyst. In Paludicella they acquire their maximum in development and constancy; they occur here between every cell, and consist of an annular process, which projects transversely from the ectocyst into the interior of the cell, with a covering of endocyst on its upper and under surface (Pl. X, fig. 4, 5). The septum thus formed is rendered complete by the aperture in its centre being closed by a peculiar body, which projects into the cavity of the cell at each side. The structure of this body is remarkable ; it consists of a central mass, surrounded by a distinct layer of somewhat elongated cellules placed perpendicularly to its surface. The body which thus closes up the centre of the annular septum has, without doubt, some office to perform besides that of simply completing the septum ; but upon the nature of this office, or the exact signification of the body itself, I can form no satisfactory opinion. In the other genera the septa are by no means so constant or complete as in Paludicella. In several species of Plumatella, especially P. coralloides (Pl. VII, figs. 2, 3), septa exist, but these generally occur only at intervals, leaving several cells between them, which communicate freely with one another: not unfrequently the septum
* Dumortier, Recherches sur l’Anat. et Physiol. des Polypes Comp. d’eau douce. “Bul. de l'Acad. Roy. de Bruxelles,’ 1835. Fig. reproduced in Dumortier and Beneden, Hist. Nat, des Pol. Comp. d’eau douce. ‘Mém. de l'Acad. Roy. de Bruxelles,’ 1848. Compl. t. xvi.
t Van Beneden, Recherches sur les Bryozoaires fluviatiles de Belgique. ‘Mém. de l'Acad. Roy. le Belg.,’ 1848.
itself is imperfect, admitting of a communication through its centre between two neighbouring cells. In Alcyonella sun/osa, and in Fredericella sultana, imperfect septa may here and there be observed, while Crissatella and Lophopus would seem to be quite deprived of them, the cells in these genera all opening into one another.
The ectocyst of the fresh-water Polyzoa appears in every case to be absolutely structureless. The fact of cellulose being a constituent of the test of the Tunicata, induced me to look for it in the ectocyst of the Polyzoa, but I have never succeeded in obtaining satisfactory evidence of its existence in any of the tissues of the Polyzoa, either fresh-water or marine. I observed, however, that in one instance the ectocyst of Plumatella repens, after lying for several weeks in a concentrated solution of caustic potash, in which it had been first boiled, presented under the microscope, at one or two points, a distinctly blue tint on being wetted with tincture of iodine, and then with sulphuric acid, a fact which would seem to point to the possibility of the true cellulose reaction being only masked by the presence of other constituents, as we know to be the case in some of the tissues of plants which fail to strike a blue colour with iodine and sulphuric acid, until the removal, by a somewhat tedious process, of the adventitious matter. I do not, however, lay any stress on the blue colour produced in the above instance, as it did not occur in others, and was therefore probably accidental. Upon the whole, the reactions of the pergamentaceous ectocysts of Plumatella, &c., are rather in favour of this tissue being composed of chisine. It is quite insoluble in strong acetic acid, and in a concentrated solution of caustic potash, even when exposed to prolonged boiling in these fluids, or after month-long maceration in them; but it is soluble in concentrated nitric, hydrochloric, and sulphuric acid. Successive boiling in water, alcohol, ether, acetic acid, and caustic potash, renders it nearly colourless, without in any way altering its form, all which properties are among the essential characteristics of chitine, which would thus seem to replace in the entocyst of the Polyzoa the cellulose of the test of the Tunicata, unless more elaborately conducted researches shall prove the essential constituent of the tissue in question be identical with the more highly nitrogenized conchiolin of ordinary Mollusca.”
(2.) Oryans of Digestion.
The digestive system is very similar in all those species in which the lophophore is bilateral (P/ylactolaemata); these we shall therefore consider together; Paludicella, which, with the exception of Urnatella (?), is the only fresh-water representative of the division with orbicular lophophore (Gymnolaemata), presents some peculiarities, and should be examined by itself.
* See Schlossberger, Zur nåheren Kenntniss der Mushelschalen, des Byssus und der Chitinfrage ‘Ann. der Chem. und Pharm.,’ xcviii, 99.
The mouth (Pl. II, fig. 24, d) is a simple edentulous orifice of a circular or slightly crescentic form, placed in the centre of the body of the lophophore, and consequently occupying the bottom of the tentacular crater. Its margin is slightly elevated, and is continuous on the neural side, with a holow valve-like organ (e) of very peculiar formation. This organ arches over the mouth, and may be aptly enough compared in shape to the epiglottis of certain mammifers. The cavity in its interior communicates through an opening (e') in the lophophore with the perigastric space; the walls which are turned towards the mouth are thick, and densely clothed on their external surface with vibratile cilia, while those which look towards the vent are thin, membranous, and transparent, and destitute of cilia. It may be observed, when the polypide is exserted from its cell, to be in a constant motion, which consists in an alternate elevation and depression of the organ. The elevation is effected by distinct muscular fibres (n), which are visible through the transparent walls, and will be afterwards more particularly described, while the depression is probably the result of an antagonistic elasticity. I propose to designate this organ by the name of epistome. On its true function I am unable to throw any light; though it is here described in connection with the organs of digestion, its relation to the digestive system is perhaps very remote. It may possibly be more correctly viewed as connected with sensation. Its homological import will be afterwards considered. From the mouth an aesophagus (Pl. II, fig. 24; V, fig. 5; IX, fig. 7, f) of considerable length leads downwards to the stomach; it becomes gradually narrower as it approaches the latter, into which it opens by a very distinct conical projection (Pl. III, fig. 7, f). To the oesophagus immediately succeeds the stomach, without the intervention of any distinct gizzard, such as we find in Bowerbankia and certain other marine Polyzoa ; and I cannot explain the statement of so excellent an authority as Siebold, who asserts that he has seen in Alcyonella a gizzard with an organization precisely similar to that of Bowerbankia.” The stomach is a large thick-walled sac, and may be divided into two portions, first a nearly cylindrical prolongation (Pl. V, fig. 5; IX, fig. 7, 9), which by one extremity immediately receives the oesophagus, while by the other it is continuous with the remaining portion of the stomach; it may be called the cardiac cavity of the stomach. The second division (/) forms the greater portion of the stomach; it is also of a nearly cylindrical form ; but it is longer and wider than the cardiac cavity with which its axis is nearly continuous; it terminates below in a rounded cul-de-sac ; to distinguish it from the other, I shall call it the pyloric cavity of the stomach. Between the cardiac and pyloric cavities there is no marked line of demarcation, the structure of both being quite similar ; notwithstanding, however, the similarity of structure, I believe there are physiological grounds for the distinction, for I consider the cardiac cavity as the true homologue of the gizzard in Bowerbankia. On a level with the continuation of the cardiac into the pyloric cavity arises the intestine (/); it springs from the pyloric cavity, with which it communicates by a very defined orifice (Pl. III, fig. 7, h'). The structure of the pylorus is such as to admit of the orifice
* “Lehrbuch der Vergleichenden Anatomie,’ $38, Note 1.
being dilated or contracted, or even completely closed. The intestine is very wide at its origin, and passes up along the side of the cardiac cavity and oesophagus, rapidly diminishing in diameter till it terminates in a distinct anus by perforating the tentacular sheath just below the mouth, and at the concave side of the lophophore.
In Paludicella articulata, the mouth (Pl. X, fig. 5) is a perfectly circular orifice, with slightly projectile margin, and is totally destitute of the valve-like appendage which, unless Urnatella should prove an exception, is found in all the other fresh-water species. The upper part of the oesophagus (fig. 3, f) is wide, and may perhaps here, more decidedly than in the other species, be distinguished as pharyna. It soon contracts into a long narrow tube, which leads to an oval sac (/) corresponding to the cardiac cavity of the stomach in the other fresh-water Polyzoa, and to the gizzard in certain marine species. This sac is much more distinct from the great cavity of the stomach than in the other Polyzoa of fresh water. It enters this cavity near its upper extremity, and presents here a well-marked constriction; in extreme retraction of the polypide it is bent back upon the rest of the stomach. The great cavity (/) of the stomach is of a nearly cylindrical figure; from its upper extremity arises the intestine (/). This tube presents, just after its origin, a wide dilation, and then suddenly contracting, continues as a narrow cylindrical canal to its termination just below the mouth.
The histological structure of the alimentary tube in both the phylactolaematous and gymnolaematous forms is somewhat complex. It may, however, be easily enough made out in the different genera. Wherever I have had a good opportunity of examining it, I have succeeded in detecting in the stomach three distinct layers. Internally is a yellowish-brown layer (Pl. III, fig. 7 k), which is thrown into large longitudinal rugae. These rugae become less decided towards the fundus of the stomach, and in Paludicella they are entirely absent. This internal layer is composed of easily separable spherical cells (Pl. II, figs. 6, 7), containing a colourless fluid, in which floats a secondary cell, with yellowish-brown contents. When the animal has been left long without food, the brown matter disappears from the cells, and the stomach becomes colourless. The inner layer of the stomach is thus distinctly glandular, and may be fairly viewed as the representative of a liver, the cells with brown contents being manifestly true secreting cells, destined for the elaboration of the bile, and capable of being set free by the rupture of the cell which encloses them.
This layer passes externally into a more compact layer (Pl. III, fig. 7, A) composed of smaller, simple cells (Pl. II, fig. 8), with colourless contents and a brilliant nucleus.
The third or most external layer is a thin membrane (Pl. III, fig. 7, u); it possesses also an undoubted cellular structure; it admits of being traced uninterruptedly over the whole tract of the alimentary canal, and, on the application of acetic acid, becomes frequently raised from the subjacent layer. Delicate circular striae may generally be distinctly observed in it, they may be seen surrounding the stomach, and are probably muscular fibres; they are particularly
evident in Alcyonella and Plumatella towards the fundus of the stomach; they become less distinct as we ascend toward the oesophagus, and totally disappear from this tube and from the rectum. The fundus of the stomach appears to differ from the rest of the alimentary canal in structure and function; the well-defined longitudinal rugae and deep brown colour of the internal layer of the stomach nearly disappear in it, and, during the process of digestion, we may perceive that the peculiar peristaltic action of the walls is more marked in it than in any other part of the gastric cavity, while it is every now and then separated from the rest of this cavity by a momentary hour-glass constriction. In the oesophagus there are only two layers (Pl. III, fig. 7, X′, u'). These correspond to the middle and external layers of the stomach, the former being here largely developed (A'), while the internal or hepatic layer of the stomach is entirely absent, and there are no longitudinal rugae. The mouth and upper part of the oesophagus are in all the genera clothed with vibratile cilia, but I could detect no appearance of cilia further than a short distance down this tube. The structure of the intestine closely resembles that of the oesophagus; vibratile cilia however, are altogether absent. In Crissatella, the cells of the internal layer corresponding to the middle layer of the stomach are large, and filled, in the well-fed animal, with a clear greenish-blue fluid. With the exception of the mouth and upper portion of the oesophagus, no part of the alimentary canal is ciliated in the phylactolaematous fresh-water Polyzoa. In Paludicella however, the stomach in the immediate neighbourhood of the pyloric orifice is lined with long vibratile cilia (Pl. X, figs. 3, 4), by which portions of alimentary matter, pushed onwards by the peristaltic contractions of the stomach, are kept in a constant state of active rotation previously to their being delivered into the intestine. The entire tract in all the genera examined is highly irritable, the presence of alimentary matter stimulating it to rapid and vigorous contraction. The whole course of the alimentary matter, from the moment of its prehension to its final ejection, may be easily witnessed in many of the fresh-water Polyzoa. If a polypide of Plumatella repens be watched while in an exserted state, different kinds of Infusoria and other minute organic bodies may be observed to be whirled along in the vortices caused by the action of the tentacular cilia, and conveyed to the mouth, where many of them are at once seized and swallowed, and others rejected. The food having once entered the oesophagus, experiences in this tube no delay, but is rapidly conveyed downwards by a kind of peristaltic action, and delivered to the stomach; and at the moment of the passage of the alimentary matter from the oesophagus into the stomach the cardia may be observed to become more prominent. In the stomach the food is destined to experience considerable delay; it is here rapidly moved up and down by a strong peristaltic action, which first takes place from above downwards, and then inverting itself, propels the contents in an opposite direction. Every now and then the fundus of the stomach, which, as has already been said, seems to perform some function distinct from that of the rest of the organ, seizes a portion of the alimentary mass, and retains it for a moment by an hour-glass restriction separate from the remainder, and then powerfully contracting on it, forces it back among the other contents of the stomach. All this time the food is becoming imbued with the peculiar secretion of the gastric walls, and soon assumes a rich brown colour. After having thus undergone for some time the action of the stomach, the alimentary matter is delivered by degrees into the intestime, where it accumulates in the wide pyloric extremity of this tube. After continuing here for a while in a state of rest, and probably yielding to the absorbent tissues its remaining