being formed by the ramifications of the branchial artery, and the other by those of the vein. These minute ramifications can be ultimately traced to two great trunks which run along the back of the branchial leaf, one (the arterial) trends away to the auricle of its own side, to pour into it the renovated blood, while the other is the large vein from which the venous branchlets have departed.*

The naked acephalous Mollúsca (Tunicata Lam.) have two, and only two, orifices in their outer tunic, which very often open on the tips of two tubular projections, or papillæ, placed near one another. By one of these †, which is usually the highest, and encircled within its rim with one or two rows of slender tentacular filaments, or furnished with a valve, the water necessary to respiration flows into a large visceral sac, which, while it seems in part to perform the functions of a crop, affords ample space for the display of the aerating blood-vessels. The water, after being breathed, is in general expelled at the same orifice by which it was sucked in; and, notwithstanding that the observations of several naturalists seemed to prove the contrary, Cuvier was nevertheless inclined to conclude, from his anatomical investigations, that it could not possibly be expelled from the other, which is the anus. But the conclusion was erroneous; for, since attention was more particularly directed to the point, the water has been seen to be propelled simultaneously from both apertures,

Manuel, p. 128. Such of our readers as are interested in the structure of molluscous animals will not be displeased at the length of the following extract from Carus:-" It is to be remarked, farther, of the large branchial laminæ of the freshwater muscle, that both pairs consist of an intertexture of vessels, arranged in a rectangular latticework, and covered by a delicate membrane, whilst the two external are distinguished by a structure which merits a particular description. Above each external lamina of the gills is a duct proceeding from the posterior part of the foot towards the anal tube, long ago described as an oviduct by Oken, and having on its lower surface a long row of openings placed transversely, and forming the entrances to the cells, or compartments, of the gills themselves. These compartments are all arranged vertically in the gill, and separated from each other by partitions: they appear as though they originated from the mutual recession of the two membranous surfaces of the gill, which remain connected only by the vertically disposed vessels that give rise to the septa: they serve for the reception of the ova, which, coming from the ovary placed within the foot, and not by any means formed in the gill itself, are, however, lodged there; and there receive their farther developement, as in a uterus. This is a remarkable instance of the connection between the sexual and respiratory functions." (Comp. Anat., vol. ii. p. 148, 149. trans.)

+ In all the compound Mollúsca, the branchial orifice of the component individuals tends always to approach to the circumference of the system, as the anus does as invariably to the centre. (Savigny.)

by more than one competent observer; and if I may not rank myself among these, yet have I witnessed the fact in the Ascídia prùnum. You will not dispute the claim of our friend Dr. Coldstream to be numbered among them. "It has been doubted," he says, "whether the Ascídiæ, in contracting their tunics, expel the water through their anal as well as through their branchial orifices. I have distinctly seen this species (Ascídia prùnum), as well as others (in particular the A. intestinalis), propel currents of water through both orifices at every contraction of the tunics; that from the anal orifice being almost as strong as the one from the mouth of the branchial sac." (Edin. Phil. Journ., October 1830, p. 240.) But, indeed, long before this, and even previously to the publication of Cuvier's memoir, Carus had detected" a lateral opening furnished with valves," in the sac, by which the water might have egression, and which, says this most ingenious anatomist, satisfactorily explained how these animals have the power of rejecting the respired water not only through the mouth, but also through the anus." (Comp. Anat., vol. ii. p. 146. trans.)

The branchial cavity itself is a large flattened sac, which varies greatly in respect of extent, depth, and form. Sometimes, as in Ascídia clavàta, it occupies only a small portion of the length of the body; oftener, as in Ascidia microcosmus, it occupies all the length and the breadth of one of the sides, and the rest of the viscera occupies the other side: then its form is oblong, oval, or rectangular. Sometimes, as in Ascídia mammillata and monàchus, after having descended even to the bottom of the outer tunic, it bends upwards until its base is at the middle of its length, and looks towards the entrance. In the latter case, the parietes have the greatest extent. In general these are smooth, and without plaits; but in some species, and, as it would appear, in all those which have a coriaceous outer cloak, they are creased into deep and regular folds, the first vestiges of the four branchial leaves of bivalves.*

* So says Cuvier, and there is no man whose opinions I so much value and respect; yet I must acknowledge that there appears to me to be no relationship whatever, either in this or any other genus, between the branchial sac of the Mollúsca tunicàta and the gills of the bivalves. In the former, the water enters the sac by an orifice common to it and the stomach; in the latter, it reaches the gills by a large fissure, or by a tube always distinct from the mouth: in the former, the aerating vessels are spread on the sides of a portion of the alimentary canal; in the latter, they form free and independent organs: in the one, they are strictly internal ; in the other, they are properly exterior to the body. Lamarck even considers

Whatever may be, however, the shape and general disposition of the sac, the texture of its inner parietes remains essentially the same, and is so very remarkable that several authors, who knew not its purport, have expressed astonishment at its beauty. It consists of an infinity of little vessels which cross one another at right angles, and thus weave a network, with quadrangular meshes (fig. 30.), that are again

subdivided by vessels of such tenuity that they elude the unaided vision, and require the microscope for their discovery. With a little attention it may be perceived that the vertical vessels come from the transverse vessels, and that these are connected by their two extremities to two great trunks, also vertical, which occupy one of the sides, or rather the edges, of the sac; and it is natural to conclude that one of these trunks is the artery, and the other the branchial vein. (Cuvier, Memoirs, xx. p. 11, 12.).

The meshes of this branchial network are generally, as I have said, nearly square and uniform, yet in the different genera there

is exhibited a considerable variety of patterns, some of which

it as a point still undetermined, whether the network of the tunicàta is vascular or not: he thinks it may be formed of muscular fibres. (Hist. Nat., vol. ii. p. 86.)

you have here copied from the beautiful plates of Savigny. Fig. 31. exhibits a small portion of the branchial surface of the Ascídia pedunculata (Boltènia ovífera Sav.), highly magnified, and is an example of its usual and least ornamental conformation; in the Ascídia (Cynthia Sav.) mytilígera the meshes are elliptical (fig. 32.); and they have the same form in many other species, more particularly in the compound families, or those in which a great number of individuals are united together in a common system. (fig. 33.) Again, in

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hespèrium.

33

Pattern of the branchia in Phallùsia sulcata.

some genera, of which the Phallùsia of Savigny is one, there is a small conical process at each angle of every little square (fig. 34.); but it is in the Cynthia Diòne that the most remarkable modification of this structure appears. Here the bran

Pattern of the branchia in Polyclinum chial tissue is not continuous upon the folds of the sac, but interrupted, at equal distances, in a manner to resemble a series of very regular festoons. Each fold has a second at its base, which is not free like itself, and of which the points of fixture correspond to the intervals which separate the festoons. The whole of the plaits are twenty-eight, fourteen on each side, and they are margined by an equal number of great longitudinal vessels. The vessels which compose the tissue are excessively fine; the transverse, however, less delicate than the others, and not so closely set, accommodate themselves very well by their curvature to the outline of the festoons. This description, I feel, needs the aid of Savigny's figure, of which I gladly avail myself, and I am certain that in few other creatures will you find a structure more wonderfully fashioned. (fig. 35.) (Mém. sur les Animaux sans Vertèbres, 2de partie, passim).

Although the branchial tissue apparently covers the whole inner surface of the sac with a continuous network, yet it is

really divided into two halves by a furrow in which the trunks of the blood-vessels lie; and this structure becomes obvious in some families where, as in Pyrosòma, the interspace is considerable; and is still more remarkably obvious in the Sálpæ, in which, in fact, the branchial vessels are not disposed on the walls of the sac, but occupy the margin of two narrow linear leaflets of very unequal lengths that lie across the

35

Cynthia Didne Savigny.

cavity. These are formed by a duplicature of the inner tunic, and the superior margin is garnished with a close series of little vessels which run parallel to one another in a transverse direction; a form and disposition which, says Lamarck, has very little analogy with what is regarded as the respiratory organ in the Ascídiæ (Hist. Nat., vol. iii. p. 114.); but which, on the contrary, Carus seems to think

appear

is just the link that connects these with the bivalves, " ing to constitute the transition from the Ascidia to the Terédines, where there are two elongated branchial laminæ above the intestine and within the tubular cloak, to which the water has access and egress by means of two tubes placed at the posterior extremity of the body." (Comp. Anat., vol. ii. p. 147. trans.)

Let us, before proceeding, reflect a moment on that wonderful diversity in the structure of the same organ here exhibited to us in one class of animated beings: it is a fine example, among many, of that variety in which the Creator of all has seen good to indulge in the production of his works, as if, to use the words of a favourite author, He " willed to show those whose delight it is to investigate his works, by how many varying processes he can accomplish the same end." I see in it also a proof, that neither external and physical circumstances, nor self-born desires, have that great and