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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 hol 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 (é) 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 æsophagus (Pl. II, fig. 24; V, fig. 5; IX, fig. 7,8) 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 æsophagus, 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 (); 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 esophagus, 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 esophagus (fig. 3,5) is wide, and may perhaps here, more decidedly than in the other species, be distinguished as pharynx. It soon contracts into a long narrow tube, which leads to an oval sac (9) 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 (9) 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 phylactolæmatous and gymnolæmatous 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 rugæ. These rugæ 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, 1) 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, x); 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 striæ 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 æsophagus, 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 rugæ 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', r'). 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 rugæ. The mouth and upper part of the æsophagus 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 esophagus; vibratile cilia however, are altogether absent. In Cristatella, 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 phylactolæmatous 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 æsophagus, 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 æsophagus 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 intestine, 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
nutritious elements, portions in the form of roundish pellets become separated at intervals from the mass, and are slowly propelled along the tube towards the anus, where, having arrived, they are suddenly ejected into the surrounding water and rapidly whirled away by the tentacular currents. It was these excrementitious pellets that Turpin mistook for unarmed ova in Cristatella.
(3.) Organs of Respiration and Circulation.
Upon the tentacular crown and the walls of the perigastric space would seem, among the Polyzoa, chiefly to devolve the function of bringing under the influence of the aërating medium the nutritious fluid of their tissues.
The tentacular crown of a Polyzoon consists of two portions, namely, first, a sort of stage or disc (the lophophore) (Pl. II, fig. 24; IX, fig. 7; X, figs. 3, 4, k) which surrounds the mouth ; and secondly, a series of tentacula (1) which are borne upon the margin of the lophophore. The lophophore is throughout almost the entire class of an orbicular figure; but in the fresh-water genera Cristatella, Lophopus, Plumatella, and Alcyonella, its neural margin, or that which corresponds to the side of the rectum, is extended into two long triangular lobes or arms, so as to cause the lophophore in these genera to present the form of a deep crescent, round whose entire margin the tentacula are borne in one continuous series. This condition of the lophophore is found in no marine species. In Fredericella the arms of the crescent are obsolete, and the lophophore here may, on a superficial view, appear orbicular; but a careful examination will render manifest its departure from the orbicular form, the side corresponding to the arms of the crescent being slightly prolonged, while the bilaterality is rendered still more decided by the presence of an epistome. Paludicella is the only fresh-water genus, if we except Urnatella, not yet sufficiently examined, in which not the slightest trace of bilaterality can be detected (Pl. X, fig. 5). With the exception of Pedicellina, which presents a condition of the lophophore altogether unique and exceptional,* and some other marine genera in which a slight tendency to assume a bilateral form may be observed, but which never possess an epistome, the lophophore in the marine genera is always orbicular. The lophophore in all the genera forms the roof of the perigastric space; in the species with crescentic lophophores the arms of the crescent are tubular and open into this space; the interior of the arms is clothed with vibratile cilia.
* The structure of Pedicellina has been hitherto entirely misunderstood. I have had an opportunity of carefully examining this Polyzoon, and as it is constructed on a plan quite peculiar, and may tend to throw light on the morphology of other types, it may not be out of place to give a description of it here.
The general appearance of Pedicellina is that of a gymnolæmatous Polyzoon with its nearly spherical cell seated on the extremity of a long peduncle. The gymnolæmatous character, however, is wholly deceptive, and depends on a remarkable condition of the lophophore, which, though at first sight orbicular, is truly bilateral and phylactolaematous, but nevertheless constructed on a type peculiar to itself, and of which no other Polyzoon affords any example, unless further examination shall show a similar disposition in Urnatella.
The two arms of the lophophore have the tentacula confined to their outer margin. They are
The tentacula are tubular, closed at their free extremity, and opening by the opposite through the lophophore into the perigastric space; in all the Polyzoa they are armed upon their opposed sides with vibratile cilia, arranged in a single series, and vibrating towards the remote extremity of the tentacle upon one side, and towards the base on the other. Two very distinct layers (Pl. IX, fig. 5) enter into the structure of the tentacula. The external layer consists of rounded cells filled with a colourless fluid, and often presenting a bright nucleus. Some of those cells which lie upon the back of the tentacle become in certain genera enlarged, giving a vesicular appearance to the organ ; this is particularly evident in Cristatella. The internal layer is a delicate transparent membrane, in which I could detect no trace of
approximated at their extremities, and thus, instead of constituting an open crescent, they form a ring enclosing a space which embraces within it the termination of the intestine.
Plan of PEDICELLINA.
Fig 4. Front view of lophophore and contiguous parts. Ectocyst. 6. Endocyst. c. Invaginated portion of endocyst adherent to the bases of the tentacula and entering into the formation of the cup. d. Muscular fibres surrounding the margin of the cup.
e, Mouth. f. Esophagus. 9.
Stomach. h. Rectum. i. Anus. k. Epistome. l. Ganglion. m. Generative organ. n. Retractor muscle. 0. Lophophore.
The tentacula are connected to one another at their base by a membrane which adheres to their hack, extending forwards for about one third of their length, and constituting a cup. This membrane passes uninterruptedly from the extremity of one arm of the lophophore to that of the other, thus binding together the two tentacula which spring from the points of the arms, and completing the tentacular crater so as entirely to disguise the hippocrepian character, and give to the crown of tentacula the form presented by the infundibulate genera. The result of this arrangement is, that the anus presents the anomalous condition of opening within the tentacular crater. It will be easily seen, however, that the position of the anus though within the circle of tentacula is still properly external to the lophophore, and thus really occupies its normal position in the concavity of the crescent.
The membranous cup which surrounds the base of the tentacular crown is not homologous with the calyx of the ordinary hippocrepian Polyzoa, but would seem rather to represent a permanently invaginated portion of the endocyst, with which, however, a true calyx equivalent to that of the fresh