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Tunicata lived entirely upon vegetable organisms. contents of the stomachs of the Phallusiæ, Clavelinæ, and Diazonæ, examined by them, consisted of particles of florideous algæ, which had probably found their way there by chance, and a great quantity of microscopic plants of low position in the series, species of Navicula, Frustulia, Baccilaria, Closterium, &c.* I think that I have also detected the remains of entomostracous insects in the sac of some Tunicata; and Savigny, who has frequently made the same observation, has found in even the compound species, crustacea of a higher order and greatly larger dimensions. The latter, however, as Cuvier thinks, may have entered against the will and to the prejudice of the mollusk; for he has observed the delicate texture of the viscera torn and ruptured by such rude ingesta.†

Of the Tunicata there are two families: one, Alcyoneæ, or the compound, in which numerous individuals, generally of very small size, are united together, and, as it were, immersed in a common somewhat gelatinous mass; and another, Ascidia, or the solitary, in which every individual is single and separate, and of much greater magnitude. In both of these families, there is a circular aperture, raised a little above the surface of the common integument or sac, and capable of being shut or opened, more or less widely, at the pleasure of the animal. The rim of it is sometimes plain, and sometimes cut into four, six, or eight equal segments; and within the orifice there is, in very many of them, a fringe formed of one or two rows of delicate cilia, which I have observed, in the Ascidia rustica, to be in con

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Forbes and Hanley, Brit. Mollusca, i. 7.

+ Mem. xx. 14-"It would seem that the food of Ascidians consists of very minute particles of organised matter; for, although small crustacea and other animal remains have been occasionally met with in the branchial chamber, nothing of this nature has been observed in the stomach itself, and, as must be obvious to the reader, the oral aperture seems but little adapted to the deglutition of bulky substances."-JONES's Anim. Kingd. p. 372.

*

stant and quick vibration when the animal was left undisturbed. I presume them to be organs of a very delicate irritability, perhaps of taste; and that their purpose is to hinder the ingress of noxious matters, not altogether mechanically, but because the sudden contraction of the oral aperture is a necessary sequence of their unpleasant irritations.† This aperture leads directly into the branchial sac, which, besides its office of a respiratory organ, seems to perform in part that also of a stomach; for that the process of digestion commences there, seems obvious from the fact, that numerous animalcules are generally found in it, but are never to be detected in the viscera of the abdomen. At the base of this sac there is another aperture (called by Cuvier the mouth), which conducts us, through the medium of a narrow membranous tube or oesophagus, into the proper stomach, an organ always much smaller than the branchial sac, very variable in point of situation and form, generally puckered into longitudinal plaits internally, and sometimes studded with some glandular bodies; but its minute structure cannot be ascertained with any degree of accuracy. It contains, in general, only a little liquid; while the intestinal canal, on the contrary, is almost, in every instance, filled throughout with a sufficiently consistent matter, sometimes grumous, more often homogeneous, of a yellowish grey colour, and rolled into little round or egg-shaped pellets, which it behoves us not to mistake for the proper ova. This canal is usually wide, and has a flexuous course; at first descending in the common sac, and then returning upon itself, it winds along the anterior side of the branchial sac, to open outwardly by a round aperture placed near the mouth, but distinguished by its lesser prominence. In the Alcyoneæ, it is otherwise like the mouth in form and structure; but, in Ascidiæ, there is no filamentous fringe at this orifice; which is furnished, instead, either with two valvular folds, or with a simple circular plait.

"The disposition of the alimentary canal determines, in a manner perfectly absolute, the kind of food by which the animal is nourished; but if the animal did not possess, in its senses and organs of motion, the means of distinguishing the kinds of aliment suited to its nature, it is obvious it could not exist." CUVIER, Comp. Anat. i. 55. trans.

"Il est garni d'une rangée de filamens charnus, ou de tentacules trésfins, qui servent sans doute à l'animal pour l'avertir des objets nuisables qui pourroient se présenter et qu'il doit repousser." CUVIER, Mém. xx.

10.

From this circumstance Savigny infers that the more gross and indigestible parts of the food are regurgitated, as they are in some nocturnal birds of prey. Mém. sur les Animaux sans Vert. ii. 8.

In many of the solitary Ascidia, the stomach is enveloped in a large liver,* which pours the bile directly into it through several orifices; and, in others, the parietes of the intestine are also thickened by a glandulous tissue, which probably secretes some liquor essential to proper digestion: but there is no liver in the social Alcyoneæ, or only some obscure traces of it in a few species, as in Diazona violacea; to the intestine of which, a little underneath the pylorus, are appended some little greenish tubes, simple, bifid, or trifid, which, Savigny conjectures, may be hepatic. There is also an essential difference in the position of the viscera in the two families; the Ascidia have the abdominal viscera applied entirely against one of the sides of the branchial sac, beyond the base of which they do not project; on the contrary, the abdominal viscera of the Alcyoneæ are without and under the sac from which they are dependent, and often separated by a distinct pedicle, the terminal portion of the intestine being the only part which is connected with the thorax. There are, however, some intermediate species to show that this distinction is one of inconsiderable. importance in their economy.

In the bivalved mollusks, the mouth, in the form of a transverse slit, is placed at the anterior part of the animal, deeply hidden between the foot and the anterior retractor muscle in the Dimyaria, and under a kind of cowl formed by the mantle in the Monomyaria. There is thus no necessity that in these animals, the nutritive fluid should pass over the branchiæ, but it is probable that the greater part really does so, for the current that enters by the respiratory siphon is driven forward and amid the gills, whence some part of it seems to be directed to the mouth, by the motions of the cilia, and by the movements of the labial palps, of which there are two on each side. These labial appendages are triangular in shape, and very variable in size; they are scored and ciliated, particularly on the inner surface, in the manner of the branchiæ, with which their connection is often very intimate; and they are almost always very soft, and directed backwards; but, in the Nucula, they are rigid, and pointed towards the mouth, similating a sort of jaws. There are no salivary glands, for, being destitute of any hard parts about the mouth for comminuting alimentary substances, glands for pouring in a fluid to blend with the food during that operation are not wanted. The oesophagus,

* In Boltenia there is no liver.-SAVIGNY'S Mém. ii. 88. + Mem. ii. 37.

usually after a very short course, dilates into a stomach, the sides of which are perforated by the large hepatic ducts, coming from the liver, in the centre of which the stomach is imbedded. The intestine is very various in length; and its convolutions, interwoven with the liver and ovaries, are generally contained in great part in the foot. Having made its convolutions, the intestine is directed towards the heart, through the ventricle of which it commonly passes, and ends on the posterior muscle by an opening which, in some species, has a divided margin. This anus is situated between the lobes of the mantle, and opens into the tube which lies alongside and above the respiratory siphon. The liver has a follicular structure, and is distinguished among the other viscera by its green colour.*

To give you a clearer idea of the course of the alimentary canal in this class of animals, I will copy, on a reduced scale, Sir E. Home's figure of it taken from the freshwater mussel. In this figure (Fig. 54), a is the mouth, into which a bristle

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has been introduced, b the stomach, after which you will observe that the intestine makes five turns in the foot amidst the ovary, and then, as rectum, runs posteriorly along the back of the animal beneath the hinge and above the respiratory organs, passing through the midst of the heart at c, and opening at d above the posterior muscle closing the shells, beneath the small tube of the cloak. This description applies generally to most bivalves, but in the oyster the rectum

*See Garner in Mag. Nat. Hist. n. s. iii. 164. p. 127. Zool. Trans. i. 272. Deshayes in Cyclop. Mery in Hist. de l'Acad. Roy. des Sc. 1710, p. 40.

Proc. Zool. Soc. iii. pt. i.
Anat. and Phys. i. 696.

does not pass through the heart; and in the Anomia the heart lies upon the intestine. In the Ungulina, Duvernoy finds that the gut passes not through the cavity, but through the parietes of the heart, a fact which militates against the hypothesis that this curious penetration of the heart in these mollusks is to render easy a more immediate passage of the chyle into the circulating system. His own less probable conjecture is, that the contractions of the heart may aid the action of the intestines: "Il m'a semblé plus exact d'attribuer ce singulier rapprochement à la necessité d'exciter et d'aider les contractions de l'intestin pour la defecation." *

In this slight sketch I have purposely omitted to notice a very remarkable organ connected with digestion, and called the crystalline stylet. There is attached to or near the stomach a small process which Deshayes compares to the vermiform process of the coecum in the higher animals. It is filled with this stylet,-an elastic, transparent, firmly gelatinous cylinder, rounded at one end and pointed at the other. The anterior extremity of this body is attached to the parietes of the stomach by means of small extremely thin and irregular auricular processes, while the other end projects into the stomach. The use of the organ is, perhaps, still conjectural. Lister, and the anatomists of his time, sometimes speak of it as analogous to the spine of vertebrated animals, and sometimes as connected with the reproductive function. Poli believed that it served to shut up the pores by which the bile is admitted, and so to regulate the flow of that secretion into the stomach. Blainville confessed his entire ignorance of its function; and Deshayes hesitates to suggest that it may act in bruising or comminuting the food. Mr. Garner asserts it to be ❝evidently analogous to the tongue of the Patella and other cephalous mollusca." Like the tongue, it is secreted from behind, and comes forward into the stomach; and the auricular processes or membrane is analogous to the membrane always found at the end of the tongue in other mollusca. Mr. Garner's opinion of its use, therefore, seems to be the same as Deshayes, but, he says, it has another use assigned to it," the giving elasticity to the foot, or, in the Anomia, where its extremity is seen in the mantle, the preserving in its situation the free extremity of the left lobe of the latter part." It may lessen the credit due from us to Mr. Garner's ingenious analogy, to remember that the stylet is not exclusively confined to these tongueless

* Ann. des Sc. Nat. (1842) xviii. 118.

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