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Determination of Aspects.
Another important point, which should be settled at the very outset of our anatomical inquiries, is the exact sense in which we are to use the terms employed to indicate the different aspects of a Polyzoon. This is the more necessary as the terms used for this purpose, in the description of the invertebrate animals generally, are frequently employed in the vaguest possible way, the same term being often applied by different authors to very different aspects of the animal.
In fixing the meaning of the terms anterior and posterior we may assume the position of the mouth as indicating the region of the animal which is to be designated as anterior, while the posterior region will then be that diametrically opposite.
In fixing the dorsal and ventral regions greater difficulty is met with. Mr. Huxley, in his very ingenious and philosophic Memoirs on the Homologies of the Mollusca,” rejects the terms dorsal and ventral altogether; generalising the Molluscan form under the conception of an ideal archetype, and finding the heart occupying one side, and the great nervous centres placed upon the opposite, he gives to the former region the name of “haemal,” and to the latter that of “neural,” thus applying to the Mollusca the terms already so happily employed by Owen in his designation of the regions of the vertebrate skeleton.
These terms have the advantage of stating a simple fact, and of thus avoiding the ambiguity which so often attaches to the terms dorsal and ventral. I shall, therefore, willingly adopt them in the present Memoir, and notwithstanding an apparent contradiction in designating as “haemal” any portion of an animal totally deprived of a blood-vascular system, I shall call that region of a Polyzoon on which the nervous ganglion lies the “neural,” and the opposite region, that, namely, which corresponds to the part of an Ascidian which contains the heart, the “haemal.”
Tabular view of the Orders and Sub-orders of Polyzoa.
The reader will be further assisted in the anatomical inquiry in which we are now about to be engaged, by having placed before him here the following scheme of the orders and sub-orders under which all the species of Polyzoa, both marine and fresh-water, admit of being arranged :
* Phylactolaemata (from pv)&ago, to guard, and Xaiua, the gullet, in allusion to the epistome placed at the entrance of the alimentary canal) corresponds in part with the Hippocrepia of Gervais. The Hippocrepia of the French zoologist, however, constitute in reality an artificial group. Being essentially characterised by the possession of a crescentic lophophore, they necessarily exclude not only Pedicellina, but even Fredericella, whose relations with the species furnished with a crescentic lophophore are of the most intimate kind. Hippocrepianism, therefore, though of great interest as a morphological fact, tending, as will be afterwards shown, to throw much light on certain homological questions, cannot be employed as the determining character of groups more comprehensive than those of generic rank.
+ Gymnolemata (from yuuvoc, naked, and Aaiua, in allusion to the absence of an epistome) corresponds to part of the Infundibulata of Gervais.
f For the structure of Pedicellina, see Note, p. 19.
§ The location of Urnatella among the Gymnolaematous Polyzoa must for the present be viewed as a provisional expedient, subject to alteration as its structure becomes better known. See the description of the genus farther on.
| The terms Cyclostomata, Ctenostomata, and Cheilostomata, were proposed by Busk, to indicate the primary subdivisions of the marine Gymnolaematous Polyzoa. (Voyage of “The Rattlesnake,” vol. i., Appendix, p. 346.)
Canacium.—The Polyzoa being all composite animals, the coenoecium 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 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 coenoecium, 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 coenoecial 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 coenoecium, but more closely approximated towards the orifices. These cells are filled with a perfectly colourless and transparent fluid. 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 oesophagus.
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 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 (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 ectocyst would seem to be entirely absent; and this genus. therefore, presents the anomalous condition of having the coenoecium composed exclusively of the endocyst.”
* This view of the coenoecium 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