Page:EB1911 - Volume 27.djvu/398

Rh and directly continuous with that layer through the atrial aperture (fig. 6); consequently the mantle is covered both externally and internally by ectodermal cells.

There is no true body cavity or coelom in the mesoderm; and yet the Tunicata are Coelomata in their structure and affinities, although it is very doubtful whether the enterocoele which has been described in the development is really found. In any case the coelom if formed is afterwards suppressed, and in the adult is only represented by the pericardium and its derivatives and the small cavities of the renal and reproductive organs.

The branchial aperture (mouth) leads into the

branchial siphon (buccal cavity or stomodaeum), 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. 5 and 6). This branchial sac is an enlarged and modified pharynx, and is therefore properly a part of the alimentary canal. The oesophagus opens from it far back on the dorsal edge (see below). The wall of the branchial sac is pierced by a large number of vertical slits—the stigmata—placed in numerous transverse rows (secondary or subdivided gill-slits.) These slits place the branchial sac in communication with the peribranchial or atrial cavity, which lies outside it (fig. 6). Between the stigmata the wall of the bronchial sac is traversed by blood-vessels, which are arranged in three regular series (fig. 7)—(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 papillae. 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.

Along its ventral edge the wall of the branchial sac is continuous externally with the mantle (fig. 6), while internally it is thickened

to form two parallel longitudinal folds bounding a groove, the “endostyle” or ventral furrow (figs. 5, 6,8, end.) corresponding to the hypopharyngeal groove of Amphioxus and the median part of the thyroid gland of Vertebrata. The endoderm cells which line the endostyle are greatly enlarged at the bottom, where they bear very long cilia, and on parts of the sides of the furrow so as to form projecting glandular pads (fig. 8, gl.). It is generally supposed that this organ is a gland for the production 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. It has, however, been pointed out that there are comparative few gland cells in the epithelium of the endostyle, and that it is possible that this furrow is merely a ciliated path along which the mucous secretion (produced in part by the subneural gland) is conveyed posteriorly along the ventral edge of the branchial sac. There are sensory bipolar cells in the lateral walls of the endostyle. 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 roove), and then join to form the anterior end of a fgold which runs along the dorsal edge of the branchial sac as far as the oesophageal aperture. This fold is the dorsal lamina (figs. 5, 6, dl).