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A. Organs for the Preservation of the Individual.
(1) Dermal System.
Cænæcium.—The Polyzoa being all composite animals, the cænæcium constitutes essentially an assemblage of little cells or chambers, of very various form, organically connected with one another. Each chamber lodges a polypide, and its cavity is either shut off from those of the neighbouring chambers, or freely communicates with them. In
In every instance* the polypide can be protruded from its cell, and again withdrawn into it, and the part through which it thus passes outwards and inwards, as has already been defined, is the orifice of the cell. It must not, however, be supposed that there is here any proper orifice, the retraction and exsertion consisting merely in an invagination and evagination of the anterior part of the cell.
The cænæcium, in every case, except in Cristatella, is composed of two distinct membranes, which must be carefully distinguished from one another—an internal, the endocyst, which is always soft, transparent, and contractile ; and an external, the ectocyst, which varies greatly in character in the different genera.
The endocyst (Pl. III, fig. 7; V, figs. 5, 6; IX, fig. 7; X, fig. 4, a) lines the interior of the cells, and when it arrives at their apertures would protrude beyond the ectocyst were it not that it here becomes invaginated or inverted into itself, and then terminates by being attached round the base of the tentacular crown. During the exsertion of the polypide, the invaginated portion of the endocyst is carried out with the latter, thus undergoing a process of evertion, which, however, in all the fresh-water species, is but partial, a portion of the endocyst, as we shall afterwards more particularly see, remaining in a permanently inverted condition ; in this respect differing remarkably from the marine species, in which the evertion of the endocyst is, perhaps, in all cases, if we except the anomalous genus Pedicellina, complete. The attachment of the endocyst to the base of the tentacular crown closes the cænæcial chambers externally, while the polypide is thus suspended in the midst of the fluid with which these chambers are filled.
If we examine the endocyst histologically, we shall find that it possesses a very distinct structure. In Lophopus, which is particularly well adapted for observing the intimate structure of this membrane, we find it composed of large irregularly shaped cells, widely separated from each other by an intervening substance towards the posterior part of the cænæcium, but more closely approximated towards the orifices. These cells are filled with a perfectly colourless and transparent Auid. Under the action of acetic acid each is distinctly seen to be bounded by a double outline, and to contain a large nucleus with nucleolus (Pl. II, fig. 9); the nucleus, with its nucleolus, are imbedded in the walls of the cell. The intervening substance, which, before the application of the acetic acid, appeared simply granular, is now seen to
* In Pedicellina, and apparently also in Urnatella, the power of protrusion and retraction is very imperfect, and is here limited to such change of position as is connected with a slight extension and flexion of the wesophagus.
consist almost entirely of bodies exactly resembling the nuclei of the cells; some of these intercellular nuclei contain two nucleoli, and seem to be undergoing division, while round others a young cell may be seen in various stages of formation.
These different stages may be satisfactorily followed, and afford a very interesting example of cytogenetic action. The first thing observed is the accumulation round the nucleus of a little mass of granular protoplasm. In the midst of this a minute vacuola next shows itself; this becomes the cell-cavity, and gradually increases in size with the enlarging cell; the nucleus is persistent, remaining attached to the cell-wall.
For reasons to be presently mentioned, it is highly probable that the endocyst is pervaded by a system of canals of extreme delicacy, which constitute an irregular network in its substance.
Besides the structure now described, peculiar fibres (Pl. II, fig. 10) are also developed in the endocyst. These are situated on its inner surface, where they constitute a well-marked layer composed of a network of transverse and longitudinal fibres over the whole extent of the endocyst. I have even succeeded in separating this network as a continuous layer.
There can be no doubt that these fibres of the endocyst are muscular, and that it is to their
presence that the high degree of contractility enjoyed by the endocyst of Lophopus is in a great measure, if not entirely, due. When treated with acetic acid, they are plainly seen to be composed of greatly elongated fusiform cells, having their pointed extremities in connection with one another. Each of these cells is then also seen to contain a nucleus with nucleolus (Pl. II, figs. 11, 12).
We have seen that in all the fresh-water genera a portion of the endocyst remains in a permanently invaginated state. It is probable that in all these genera the endocyst retains its general structure and contractility for a greater or less extent of its permanently invaginated portion, down to a spot where the transverse fibres appear to become condensed into a sort of sphincter, and shortly after this the endocyst alters its texture, losing its contractility and becoming thinner (Pl. V, fig. 6). In this condition it continues till it terminates by being attached to the base of the tentacular crown. This thin, non-contractile portion of the endocyst constitutes the tentacular sheath which encloses and protects the tentacula during the retracted state of the polypide. Near the spot where the endocyst passes into the tentacular sheath, there appears to exist, at least in Lophopus, a circular canal, which here passes transversely round the endocyst. The presence of this canal is revealed by peculiar, spherical or oval, brilliant corpuscles, which it almost always contains in Lophopus.
A portion, perhaps the whole, of the inner surface of the endocyst is clothed with vibratilecilia.
Though I have not succeeded in making out the structure of the endocyst in the other genera so satisfactorily as in Lophopus, we may, nevertheless, conclude that it is nowhere very far different from that now described. In all these, fibres may be detected in the endocyst. In the species with bilateral lophophore, the fibres may be seen towards the apertures of the cells (Pl. V, figs. 5, 6,v) ; but it is generally impossible, in consequence of the increasing opacity of the superjacent structures, to trace these fibres to any distance posteriorly. In Paludicella, whose transparent ectocyst admits of a distinct view of all the contained parts, the fibres are collected into numerous transverse bands (Pl. X, figs. 3, 4, v), which we may trace throughout the whole extent of the cell.
A peculiar condition of the endocyst of Lophopus, though most probably only abnormal, must be mentioned here. In specimens of this Polyzoon which had been kept for a few days, and occasionally in some just captured, multitudes of minute oval brilliant corpuscles were seen to have been developed in the endocyst throughout its whole extent. They were not scattered at random through this membrane, but were contained in the interior of a system of tubes which formed a network in the substance of the endocyst (Pl. II, fig. 13). They are about the oth of an inch in the longer diameter, larger at one end than at the other, and in the large end they appear to contain a minute cavity (fig. 14), which under the action of acetic acid dilates and fills nearly the entire corpuscle (fig. 15). The situation of these bodies in a tubular network in the substance of the endocyst is a fact of great interest.
It is nearly certain that whatever may have been the origin of the corpuscles, they found the tubes already existing for their reception. It would follow from this that the presence of a reticulated system of tubes in the substance of the endocyst is the normal condition of this tunic, but from the delicacy of these tubes, and the transparency and want of colour of their contents, they escape detection under ordinary circumstances, and are first revealed only by the abnormal (-) development of the peculiar corpuscles in their interior. These corpuscles are not confined to the endocyst, but are also found at the same time in other tissues, especially in the substance of the funiculus, which, as will be afterwards shown, connects the fundus of the stomach with the walls of the cells. In the endocyst alone, however, do they appear to be contained in distinct canals.
In Cristatella, where the endocyst constitutes the whole of the cænæcium, it presents below a flattened disc, which closely resembles the foot of a gasteropodous mollusc, and on which this singular colony creeps about on the stems and leaves of aquatic plants, exposing its beautiful plumes to the light and warmth of the sun (Pl. I, fig. 2).
The ectocyst or external investment (Pl. III, fig. 7; V, figs. 5, 6 ; IX, fig. 7; X, fig. 4, a) is, in most of the species, composed of a tough pregamentaceous brown membrane, strengthened by the deposition of irregularly formed siliceous and other earthy particles, which, except towards the orifices, where these particles are deficient, give to the ectocyst an opacity which renders an observation of the contained parts a matter of considerable difficulty. In some species of Plumatella, and in Alcyonella flabellum and A. Benedeni, the earthy particles are entirely absent, from a longitudinal line which commences wide near the aperture of the cell, and gradually narrows as it passes backwards, when it soon assumes the appearance of a prominent keel, and then loses its transparency by the deposition of earthy matter, as in the rest of the ectocyst (Pl. VIII, figs. 2, 3). The perfectly transparent wide origin of this line gives to the orifice of the cell the appearance of having a deep notch on one side. In Fredericella a slightly prominent keel is also apparent, but the notch-like transparent space does not here exist.
In Cristatella (Pl. I) the ectucyst would seem to be entirely absent; and this genus, therefore, presents the anomalous condition of having the conæcium composed exclusively of the endocyst. *
* This view of the cænæcium of Cristatella is contrary to the opinion previously expressed by me, but I am now convinced that what I formerly described as the ectocyst of Cristatella is really the endocyst.
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.†
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. 3,1"). 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 sele which crown the cell in Bowerbankia and other ctenostomatous Polyzoa ; if these setæ 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 setæ in the ctenostomatous Polyzoa being connected to one another by a delicate membrane does not in the least in validate 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, 6'). 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.
† Van Beneden, Recherches sur les Bryozoaires fluviatiles de Belgique. “Mém. de l'Acad. Roy. le Belg.,' 1818.
itself is imperfect, admitting of a communication through its centre between two neighbouring cells. In Alcyonella fungosa, and in Fredericella sultana, imperfect septa may here and there be observed, while Cristalella 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 chitine. 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.) Organs of Digestion.
The digestive system is very similar in all those species in which the lophophore is bilateral (Phylactolæmata); 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 (Gymnolæmata), presents some peculiarities, and should be examined by itself.
* See Schlossberger, Zur näheren Kenntniss der Mushelschalen, des Byssus und der Chitinfrage 'Ann. der Chem. und Pharm.,' xcviii, 99.