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Show 30 ON CLASSIFICATION. easily recognisable by the circumstance that their external integument is provided with two prominent, adjacent apertures, so that they look very 1nuch like double-necked jars (Fig. 10). At Fig. 10. first sight you might hardly suspect the animal nature of one of these singular organisms, when fresh! y 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 contraetile 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 tentades (Fig. 11, c). It invariaLly Fig. 10.-Phallusia me.n- leads into an exceeding-ly dilated pharynx tula · a oral· b atnal ' . apertu/e; a: base of the sides of which are, more or less ex ten-attachment. s1· ve 1y , per£ o rat e d. Th e gu II e t comes of f' 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 " hremal." And this hremal flexure of the intestine in the Ascidians thus constitutes an important element in the definition of the class. In these ani1nals there is an atrial system, the developmeut of which is even more extraordinary than in the Polyzoa. The THE ASCIDIOIDA. 31 second aperture to which I have referred (b, ]""'igs. 10 Rnd 11) is continued into a large cavity, lined by Fig. 11. a mmnbrane, which is reflected, like a a. serons sac, on the viscera, and constitutes what is called the "t.hird tunie," or "peritoneum." From the chamber which lies imn1ediately 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 loft lamellre 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 in distinct plications (as in the genus Pha.llusia ), 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 coinmunication with that of the atrium; and, b Fig. ll.-Phallus1·a mentula; the 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, cesophageal opening; g, stomach; 11, intestine performing its hGCmal flexure; i, anus; ll, atrium ; t, ganglion; m, eudostylc ; n, heart. |