Page:Encyclopædia Britannica, Ninth Edition, v. 23.djvu/633

Rh matrix, containing cells of various shapes; it is frequently pigmented and is penetrated by numerous lacunæ, in which the blood flows. Inside the mantle, in all parts of the body, except along the ventral edge, there is a cavity,—the atrial or peribranchial cavity,—which opens to the exterior by the atrial aperture. This cavity is lined by a layer of cells derived originally from the ectoderm and directly continuous with that layer through the atrial aperture (fig. 5); consequently the mantle is covered both externally and internally by ectodermal cells.

The branchial aperture (mouth) leads in to the branchial siphon (buccal cavity or stomodæum), and this opens into the anterior end of a very large cavity (the branchial sac) which extends nearly to the posterior end of the body (see figs. 4 and 5).

4. Diagrammatic dissection of A. mentula to show the anatomy.at, atrial aperture; br, branchial aperture; a, anus; brs, branchial sac; dl, dorsal lamina; dt, dorsal tubercle end, endostyle; h, heart; I, intestine; m mantle; ng, nerve ganglion; œ, oesophagus; œa, œsophageal aperture; ov, ovary; pbr peribranchial cavity; r, rectum; st, stomach; t, test; tn, tentacles; vd, vas deferens; ngl, subneural gland.(Original.)

This branchial sac is an enlarged and modified pharynx, and is therefore properly a part of the alimentary canal. The œsophagus opens from it far back on the dorsal edge (see below, p. 612). The wall of the branchial sac is pierced by a large number of vertical slits,—the stigmata,—placed in numerous transverse rows. These slits place the branchial sac in communication with the peribranchial or atrial cavity, which lies outside it (fig. 5, B).

5. Diagrammatic longitudinal (A) and transverse (B) sections through Ascidia to show the position of the ectoderm and the relations of the branchial and peribranchial cavities. The lettering is the same as for fig. 4. B represents a section taken along the dotted line A-B in A.(Original.)

Between the stigmata the wall of the branchial sac is traversed by blood-vessels, which are arranged in three regular series (fig. 6), (1) the transverse vessels, which run horizontally round the wall and open at their dorsal and ventral ends into large longitudinal vessels, the dorsal and ventral sinuses; (2) the fine longitudinal vessels, which run vertically between adjacent transverse vessels and open into them, and which bound the stigmata; and (3) the internal longitudinal bars, which run vertically in a plane internal to that of the transverse and fine longitudinal vessels. These bars communicate with the transverse vessels by short side branches where they cross, and at these points are prolonged into the lumen of the sac in the form of hollow papillæ. The edges of the stigmata are richly set with cilia, which drive the water from the branchial sac into the peribranchial cavity, and so cause the currents that flow in through the branchial aperture and out through the atrial.

6.—A. Part of branchial sac of Ascidia from inside. B. Transverse section of same, tr, transverse vessel; cd, connecting duct; hm, horizontal membrane; 'il, internal longitudinal bar; Iv, fine longitudinal vessels; p, p′, papillæ; sg, stigmata. A and B are drawn to different scales. (From Herdman, Challenger Report.)

Along its ventral edge the wall of the branchial sac is continuous externally with the mantle (fig. 5, B), while internally it is thickened to form two parallel longitudinal folds bounding a groove, the "endostyle," hypobranchial groove, or ventral furrow (figs. 4, 5, end}. The endoderm cells which line the endostyle are greatly enlarged at the bottom and on parts of the sides of the furrow so as to form projecting pads, which bear very long cilia. It is generally supposed that this organ is a gland for the pro duction of the mucous secretion which is spread round the edges of the branchial sac and catches the food particles in the passing current of water; but it has recently been pointed out that there are comparatively few gland cells in the epithelium of the endostyle, and that it is more probable that this furrow is merely a ciliated path along which the mucous secretion (produced possibly by the subneural gland) is conveyed posteriorly along the ventral edge of the branchial sac. At its anterior end the edges of the endostyle become continuous with the right and left halves of the posterior of two circular ciliated ridges,—the peripharyngeal bands, which run parallel to one another round the front of the branchial sac. The dorsal ends of the posterior peripharyngeal band bend posteriorly (enclosing the epibranchial groove), and then join to form the anterior end of a fold which runs along the dorsal edge of the branchial sac as far as the œsophageal aperture. This fold is the dorsal lamina (figs. 4, 5, dl). It probably serves to direct the stream of food particles entangled in a string of mucus from the anterior part of the dorsal lamina to the œsophagus. In many Ascidians this organ, instead of being a continuous membranous fold as in A. mentula, is represented by a series of elongated triangular processes—the dorsal languets,—one attached in the dorsal median line opposite to each transverse vessel of the branchial sac. The anterior peripharyngeal band is a complete circular ridge, having no connexion with either the endostyle or the dorsal lamina. In front of it lies the prebranchial zone, which separates the branchial sac behind from the branchial siphon in front. The prebranchial zone is bounded anteriorly by a muscular band—the posterior edge of the sphincter muscle,—which bears a circle of long delicate processes, the tentacles (figs. 4, 7, 8, tn). These project inwards at right angles so as to form a network across the entrance to the branchial sac. Each tentacle consists of connective tissue covered with