several genera, as the Arcade and Pecten, the branchial vessels, instead of being connected parallel to one another within the thickness of a common membrane, continue unconnected through their entire length, and they are thus formed of a great number of extremely delicate filaments attached by the base within a membranous pedicle, in which the branchial veins pursue their way towards the auricle. In a great many families and genera the branchiæ of one side have no communication with those of the opposite side; in some others, however, as in the genus Unio, the four branchial laminæ meet under the foot, and the whole of their vessels empty themselves into a venous sinus of considerable size."*

The Brachiopods, you will remember, derive their name from the fringed and curled arms which they can evolve beyond the compass of their valves; and some comparative anatomists have supposed that, whatever other use they were of, was combined that of aerating the blood. The opinion of Cuvier was different; and the beautiful dissections of Mr. Owen have proved that the true branchial vessels are ramified, in rich profusion, upon the inner surface of the lobes of the mantle, which are consequently the chief, if not the sole respiratory organ. "In this profuse distribution of vessels over a plain membranous expansion, we perceive the simplest construction of the water-breathing organ, or branchia; and, while it proves the close affinity of the Bra

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.-See also, Adanson Senegal, pref. Liv.

Deshayes in Cyclop. Anat. and Phys. i. p. 699. Mr. Garner's sketch of the variations of the branchia in the Lamellibranchiates is very interesting.-CHARLESWORTH'S Mag. N. Hist. iii. 169.

In the genus Solenom ya, Lam. there is a singular anomaly in the structure of the branchiæ. "Branchiæ duo non quatuor, non lamelliformes, sed pectinatæ vel potius pennam exacte referentes, lamellis transversis perpendicularibus, carina media corpori per totam longitudinem adnatæ, versus apicem ope ligamenti."-PHILIPPI, Mol. Sicil. i. p. 16.

M. Valenciennes has observed, that the Lucinidae and Corbis possess only a single branchia on each side of the foot and viscera; and, at the same time, the labial palpi, or accessory branchiæ, are all four wanting.-Ann. and Mag. N. Hist. xvi. 43. FORBES and HANLEY, Brit. Moll. ii. 42.—In Pholadomya and Anatina, Professor Owen had previously demonstrated that the two lamella of the same side are so united, as to appear like a single gill.-Lect. Inverteb. Anim. 283.

chiopoda to the Ascidia, it presents, at the same time, a beautiful analogy with the elementary forms of the airbreathing_organ, as it exists, for example, in the pulmoniferous Gasteropods."*

The naked acephalous mollusca (Tunicata, Lamk.) 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 vent. The conclusion is only partially correct, for it would be erroneous were we hence led to infer that the animals cannot eject any of the contained water from the vent. The water which distends the body of a healthy submerged ascidian flows into it simultaneously by both orifices in a current so still that no stream is observable. Lister was well cognisant of this fact; and Reaumur states expressly, that the water enters at times, and is driven out by either of the siphonal apertures. Spallanzani has described the phenomena, in our opinion, more accurately than any other observer.§ Dr. Coldstream says: "It has

* Owen in Zool. Trans. i. 154.-Also p. 148.

In all the compound mollusca, 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.

Hist. de l'Acad. Roy. des Sciences, 1710, 588.

"In my various excursions on the sea, I have observed that it contains many animals, which, absorbing the water by their mouth, produce a small vertiginous current, that runs into it. The animal in question has not this power. The water enters it almost insensibly, by occupying gradually the internal space which was empty. This I perceived by making use of the lens, but more distinctly by tinging the sea-water with cochineal, as the animal will live several hours in this water without any apparent injury. The red particles of the tincture will then be seen slowly to enter the two apertures with the water, gradually filling the vacuity of the animal without the appearance of a current of any kind. After some time, the slow motion of the particles ceases; that is, when the internal cavity is completely filled with the coloured water, which I could cause to issue from the apertures at pleasure, by pressing the Ascidia between my fingers.

been doubted whether the Ascidia, in contracting their tunics, expel the water through their anal as well as through their branchial orifices. I have distinctly seen this species (Ascidia prunum), 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. ""** 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." + I believe there is an error in this anatomical explanation. Mr. Garner, who knew well that the water entered the body by both orifices, says, that by one it "enters the respiratory sac, and by the other it is drawn into the external meshes of the branchiæ. The water drawn in by each opening must make its exit by the same. Those writers who say the contrary must be incorrect, unless the water pass through the stomach and intestine." In like manner the water enters the respiratory organs of some bivalves (e.g. Teredo, Pholas, Mya, and Solen) by both siphons." +

The branchial cavity itself is a large flattened sac, which varies greatly in respect of extent, depth, and form. Sometimes, as in Ascidia clavata, it occupies only a small portion of the length of the body; oftener, as in Ascidia microcos

"If, when all the water is thus brought out of the Ascidia, the animal be immersed in that contained in a vessel, in such a manner, that only one of the apertures be under water, it will fill itself completely by that, whether it be the upper or the lower. It is evident from this, that there is a communication between the two apertures; of which I also had another proof equally demonstrative, in the air which issued from the lesser aperture, and which, by means of a small tube, I could, without force, cause to pass into the greater, and vice versa. When, besides, I kept one of the apertures closed while I blew into the other, the animal swelled like a bag, and the air found no vent. "It appears therefore certain, that the upper aperture is the mouth of the animal, and the lower the anus. In fact, by the latter, I have frequently seen the Ascidia discharge matters which had all the appearance of being excrementitious. This lower orifice, likewise, communicates with another channel, or organ, as we shall see presently."-Travels in the Two Sicilies, iv. 264-6. The species on which Spallanzani made his observations is defined :"Ascidia coriacea lævis subdiaphana, apertura superiore octagona, humiliore heptagona."-p. 274, pl. 10, fig. 1-7. I believe it to be the As. prunum of Linnæus. It is not figured by Forbes and Hanley, yet I feel certain that it is a British species. Edinb. Phil. Journ., October, 1830, p. 240. Mag. Nat. Hist. n. s. iii. 170.

+ Com. Anat., ii. 146, trans.

mus, it occupies all the length and the breadth of one of the sides, and the rest of the viscera occupies the other side:

Fig. 48.

opened.

then its form is oblong, oval, or rectangular. Sometimes, as in Ascidia mammillata and monachus, 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.

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. 48), 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 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.*

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 you have here copied from the beautiful plates of Savigny. Fig. 49 exhibits a small portion of the

* Cuvier, Mem. xx. 11, 12.-Spallanzani mistook the vessels for minute muscles,-"the longitudinal to shorten by their action the length of the body, and the transversal to contract the breadth."-Travels, iv. 269.

branchial surface of the Ascidia pedunculata (Boltenia ovifera, Sav.), highly magnified, and is an example of its usual

and least ornamental conformation; in the Ascidia (Cynthia, Sav.) mytiligera the meshes are elliptical (Fig. 50); 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. 51). Again, in some genera, of which Fig. 51.

Fig. 52.

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the Phallusia of Savigny is one, there is a small conical process at each angle of every little square (Fig. 52); but it is in the Cynthia Dione that the most remarkable modification of this structure appears. Here the branchial folds of the sac, but interrupted, tissue is not continuous upon the 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.

Pattern of the branchiæ in Pyloclinum hesperium.