Imágenes de páginas

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.

b. Gymnolmmata.

In Paludicella articulata, the mouth (PI. X, fig. 5) is a perfectly circular orifice, with slightly projectile margin, and is totally destitute of the valve-like appendage which, unless TJmatella should prove an exception, is found in all the other fresh-water species. The upper part of the oesophagus (fig. 3,/) 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 {y) 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 (y) of the stomach is of a nearly cylindrical figure; from its upper extremity arises the intestine (h). 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 (PI. Ill, 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 (PI. 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 (PI. Ill, fig. 7, X) composed of smaller, simple cells (PI. II, fig. 8), with colourless contents and a brilliant nucleus.

The third or most external layer is a thin membrane (PI. Ill, 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 (PI. Ill, fig. 7, X', p). These correspond to the middle and external layers of the stomach, the former being here largely developed (X'), 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 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 phylactolaematous fresh-water Polyzoa. In Paludicella however, the stomach in the immediate neighbourhood of the pyloric orifice is lined with long vibratile cilia (PI. 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 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


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.

[graphic][graphic][merged small][merged small][merged small]

Ectocyst. b. 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. /. (Esophagus, ff. Stomach, h. Rectum, i. Anus. k. Epistome. I. Ganglion, m. Generative organ. n. Retractor muscle, o. Lophophore.

The tentacula are connected to one another at their base by a membrane which adheres to their back, 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

structure; it resists putrefaction longer than the external cellular layer, and forms the immediate walls of the tubular cavity. Nervous filaments, and certain bands, probably muscular, to be presently described, may be traced as far as the root of each tentacle, and doubtless also enter into its structure. In Cristatella, a minute cavity, which looks as if it were cut off from the rest of the tube, may be very plainly seen in the extremity of each tentacle; this condition would also seem to exist in other genera, but it is nowhere so well marked as in Cristatella.

In all the fresh-water genera, with the exception of Paludicella, and possibly of Urnatella. the entire plume of tentacula is surrounded at its base by an exceedingly delicate

water Phylactolaemata is probably united. In order to understand this relation, it is necessary to conceive of the polypide as partially retracted; the invaginated portion of the endocyst must then be viewed as adherent to the external portion externally, and to the tentacula (and calyx ?) internally.

From this arrangement it is obvious, that the exsertion and retraction of the polypide must be very limited. When the polypide desires to withdraw under cover of the cell, the free portion of each tentacle is rolled inwards as far as the margin of the calyx-like cup, and then the mouth of the cup is closed over the whole by the action of a well-developed sphincter muscle, the tentacular crown being at the same time slightly drawn backwards by some retractor fibres, which may be seen extending from the upper part of the pharynx to the base of the cell. The sphincter would seem to represent a condensed band of the parietal muscles of other Polyzoa, or it is probably homologous with the vaginal sphincter, which is nothing more than a peculiar development of these muscles in the invaginated endocyst. The retractor fibres are obviously homologous with the great retractor muscles of the others.

The mouth, which occupies its normal position in the body of the lophophore, opens into an oesophagus, which after a short course terminates in the stomach. This is a large sac which lies in the bottom of the cell; close to the pyloric orifice it becomes much constricted, and this constricted portion passes into the intestine whose origin is nearly on a level with the cardiac orifice, but separated from it by a considerable space. The intestine passes first forwards and towards the neural side, then turns towards the mouth to open after a wide dilation between the arms of the lophophore in the bottom of the tentacular crater.

The concave margin of the body of the lophophore is raised into a kind of thickened rim, which arches over the mouth, so as to constitute an epistome thickly ciliated on its oral aspect, but not provided with muscles like the epistome of the ordinary phylactolaematous Polyzoa.

The hepatic layer, which, in other Polyzoa, forms a continuous lining extending over nearly the whole of the internal surface of the stomach, is here confined to the anterior portion of this sac, in the space between the cardiac and pyloric orifices, where it constitutes a dark brown glandular mass composed of distinct cells, which are filled with the characteristic brown secretion. The rest of the stomach, as well as the oesophagus, is lined with vibratile cilia, which are especially developed in the vicinity of the pylorus.

The ovary is an irregularly shaped mass, situated between the cardiac and pyloric orifices of the stomach, and the animal appears to be unisexual, the testis occupying the place of the ovary in other individuals.

The stalk on which the cell containing the polypide is elevated, constitutes a very characteristic feature of Pedicellina. It is a tube in whose walls both ectocyst and endocyst can be demonstrated; it contains straight muscular fibres, which extend from the base of the cell to the point of attachment of the stalk; and besides these, more delicate circular fibres can also be detected in it. By the action of the straight and circular fibres, various motions, especially those of flexion and extension, can be

« AnteriorContinuar »