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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 (PI. 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 Lophopm 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 (PI. 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 (PI. 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 (PI. 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 (PI. X, figs. 3, 4, v), which we may trace throughout the whole extent of the cell.

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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 (PI. II, fig. 13). They are about the s^th 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 coenoecium, 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 (PI. I, fig. 2).

The ectocyst or external investment (PI. Ill, 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 (PI. 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 (PI. I) the ectocyst would seem to be entirely absent; and this genus, therefore, presents the anomalous condition of having the coenoecium composed exclusively of the endocvst*

* This view of the coenoecium of Cristatella is contrary to the opinion previously expressed by nu, but I am now convinced that what I formerly described as the ectocyst of Crisfafelta is really tin endocvst.

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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^

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 (PI. X, fig. 3, b"). 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 seta 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, b'). 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. 'Bui. de l'Acad. Roy. de Bruxelles,' 1835. Fig. reproduced in Dumortier and Beneden, Hist. Nat. des Pol. Comp. d'eau douce. 'Mem. de l'Acad. Roy. de Bruxelles,' 1848. Compl. t. xvi.

t Van Benedeu, Recherches sur les Bryozoaires fluviatiles de Belgique. 'Mem. de l'Acad. Rov. le Belg.,' 1848.

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a. Phglactolamaia.

The mouth (PI. 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 hoi ow 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 (»), 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 oesophagus (PI. II, fig. 24; V, fig. 5; IX, fig. 7,/) 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 (PI. Ill, 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 (PI. V, fig. 5; IX, fig. 7, g), 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 {g') 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 (h); it springs from the pyloric cavity, with which it communicates by a very defined orifice (PI. Ill, fig. 7, h'). The structure of the pylorus is such as to admit of the orifice

* 'Lebrbuch der Vergleichenden Anatomie,' § 38, Note 1.

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