we shall find, nevertheless, a singular fundamental resemblance of internal structure to the latter. All known Polyzoa are compound animals, that is to say, the product of every ovum gives rise, by gemmation, to great assemblages of partially independent organisms, or zooids. The Brachiopoda, on the contrary. are all simple, the product of each ovum not giving rise to others by gemmation. All the Brachiopoda possess a bivalve shell—a shell composed of two, more or less horny, or calcified, pieces, which are capable of a certain range of motion on one another, and are very commonly articulated together by teeth and sockets. The proper body, which is small when compared with the size of the shell, has its dorsal integument produced into broad membranous expansions, which line the interior of the valves of the shell, and are called the lobes of the mantle, or "pallium." The aperture of the mouth is situated in the middle line, between the pallial lobes, and, on each side of it, is a longer or shorter prolongation of the body, provided with ciliated tentacula. It is from the presence of these "arms" that the class has received its name. The tentaculate oral disk of a Plumatella is already horse-shoe shaped (Figs. 7 and 8); suppose each crus of the horse-shoe to be pulled out to a much greater length, and tentaculated "arms" would be produced, closely resembling those of the Brachiopoda. The mouth leads into a gullet which is directed towards, or lies along, that side of the body from which one lobe of the mantle, the anterior, is continued; the gullet opens into a stomach, provided with a well-developed liver; and from the stomach, an intestine proceeds, which is directed towards, or along, that side of the body from which the other lobe of the mantle proceeds; and then either, as I pointed out some years ago, * ends, blindly, in the middle line (Fig. 9), or else terminates in a distinct anus between the pallial lobes. * Professor Owen, in the second edition of his lectures on the "Comparative Anatomy and Physiology of the Invertebrate Animals," published in 1855, thought it not unbecoming to sneer at this discovery. "There may be blindness somewhere, but I think not at the termination of the intestine of Terebratula."-L. c., p. 403. As my statements have subsequently been fully borne out by Mr. Albany Hancock and by M. Lacaze Duthiers--two of the best minute anatomists of the day- I trust Mr. Owen is now fully satisfied as to where the "blindness" really was, in 1855. Fig. 9 neural flexure (Fig. 9). In all Brachiopoda which have been rectum; in other words, the intestine has, as in the Polyzoa, a the mouth, in the re-entering angle between the gullet and the The principal ganglionic mass is situated behind and below Fig. 9.-Lateral view of the viscera of Waldheimia australis (after Hancock). a, anterior layer of mantle : the surrounding medium, has been discovered. This, which I ated in the interior of the body, but in free communication with carefully dissected, a singular system of cavities and canals situ shall term the "atrial" system, from its close correspondence with the system of cavities, which has received the same name in the Ascidians, has been wrongfully regarded as a part of the true vascular system, and the organs by which it is placed in communication with the exterior have been described as "hearts.” There are sometimes two and sometimes four of these "pseudo-hearts" situated in that part of the body wall which helps to bound the pallial chamber. Each pseudo-heart is divided into a narrow, elongated, external portion (the socalled "ventricle "), which communicates, as Mr. Hancock has proved, by a small apical aperture with the pallial cavity; and a broad, funnel-shaped inner division (the so-called "auricle"), communicating, on the one hand, by a constricted neck with the so-called "ventricle;" and, on the other, by a wide, patent mouth, with a chamber which occupies most of the cavity of the body proper, and sends more or less branched diverticula into the pallial lobes. These have been described as parts of the blood vascular system; and arterial trunks, which have no existence, have been imagined to connect the apices of the ventricles with vascular networks of a similarly mythical character, supposed to open into the branched diverticula. In fact, as Mr. Hancock has so well shown in his splendid and exhaustive memoir, published in the Philosophical Transactions for 1857, the true vascular system is completely distinct from this remarkable series of "atrial" chambers and canals, the function of which would appear to be to convey away excretory matters and the products of the reproductive organs, which are developed in various parts of the walls of the atrial system. The precise characters of the true vascular system of the Brachiopoda probably require still further elaboration than they have yet received; and the same may be said, notwithstanding the valuable contributions of F. Müller and of Lacaze Duthiers, of their development; but the shell, the pallial lobes, the intestine, and the nervous and the atrial systems, afford characters amply sufficient to define the class. The next great division is that of the ASCIDIOIDA, which, like the Brachiopoda, are marine animals, and are very common all over the world; the more ordinary forms of them being always Fig. 10. b a easily recognisable by the circumstance that their external integument is provided with two prominent, adjacent apertures, so that they look very much like double-necked jars (Fig. 10). At first sight you might hardly suspect the animal nature of one of these singular organisms, when freshly taken from the sea; but if you touch it, the stream of water which it squirts out of each aperture reveals the existence of a great contractile power within; and dissection proves that this power is exerted by an organism of a very considerable degree of complexity. Of the two apertures, the one which serves as a mouth is often but not always surrounded by a circlet of tentacles (Fig. 11, c). It invariably Fig. 10.—Phallusia men- leads into an exceedingly dilated pharynx, tula; a, oral; b, atrial attachment. - aperture; c, base of the sides of which are, more or less extensively, perforated. The gullet comes off from the end of the pharynx, and then dilates into the stomach, from which an intestine, usually of considerable length, is continued to the anal aperture. The latter is almost always situated within a chamber, which opens, externally, by that second aperture upon the exterior of the test, to which I referred just now. Furthermore, in all Ascidians which I have examined, the first bend of the intestine takes place in such a manner that, if the intestine continued to preserve the same direction, it would end on the opposite side of the mouth to the nervous ganglion (Fig. 11); in other words, the nervous ganglion would not be situated in the re-entering angle between the gullet and the rectum, but on the opposite side of the gullet to that angle. Therefore, the flexure of the intestine is not neural, as in the Polyzoa; but as, on the contrary, the intestine is primarily bent towards the heart side of the body, its flexure may be termed "hæmal." And this hæmal flexure of the intestine in the Ascidians thus constitutes an important element in the definition of the class. In these animals there is an atrial system, the development of which is even more extraordinary than in the Polyzoa. The m d Fig. 11. a second aperture to which I have referred (b, Figs. 10 and 11) is continued into a large cavity, lined by a membrane, which is reflected, like a serous sac, on the viscera, and constitutes what is called the "third tunic,' or “peritoneum." From the chamber which lies immediately within the second aperture (k, Fig. 11) it is reflected over both sides of the pharynx, extending, towards its dorsal part, very nearly as far as that structure which has been termed the "endostyle" (m, Fig. 11). It then passes from the sides of the pharynx to the body walls, on which the right and left lamellæ become continuous, so as to form the lining of the chamber (k), into which the second aperture (b) leads, or the "atrial chamber." Posteriorly, or at the opposite end of the atrial chamber to its aperture, its lining membrane (the "atrial tunic ") is reflected to a greater or less extent over the intestine and circulatory organs, sometimes inclosing each of their parts h in distinct plications (as in the genus Fig. 11.-Phallusia mentula; the Phallusia), sometimes merely passing over them, and limiting the blood sinus in which they are contained (as in Clavelina, &c.). Where the atrial tunic is reflected over the sides of the pharynx, the two enter into more or less close union, and the surfaces of contact become perforated by larger or smaller, more or less numerous, apertures. Thus the cavity of the pharynx acquires a free communication with that of the atrium; and, test removed, and hardly more of the animal drawn than would be seen in a longitudinal section. a, oral aperture; b, atrial aperture; c, circlet of tentacles; d, pharyngeal, or branchial, sac: the three rows of apertures in its upper part indicate, but do not represent, the pharyngo-atrial apertures; e, the languets: a series of tongue-shaped processes which project into the branchial sac; f, œsophageal opening; g, stomach; h, intestine performing its hæmal flexure; i, anus; k, atrium; 1, ganglion; m, endostyle; n, heart. |