BRANCHIAL SAC.—Thus we have the dorsal lamina (or the languets) along the dorsal edge, the endostyle along the ventral edge, and the peripharyngeal bands around the anterior end. The wall of the branchial sac itself is penetrated by a large number of channels through which blood flows. Some of these run in one direction and some in another, so as to form complicated networks, which differ greatly in their arrangement in different Ascidians. Between these blood-channels there are clefts ("stigmata"), the secondary or subdivided gill-slits, by means of which the current of water passes from the branchial sac to the large external peribranchial or atrial cavity. All the stigmata (of which there may be several hundred thousand) in the wall of the branchial sac are bounded by cubical or columnar epithelial cells, which are ciliated. These cilia, so long as the animal is alive, are in constant motion, so as to drive the water onwards, and it is this constant ciliary action in the walls of the branchial sac that gives rise to the all-important current of water streaming through the body. In addition to the stigmata there are generally one or two much larger elongated slits (Garstang's {48}pharyngo-cloacal slits) placed close to the dorsal lamina and leading direct to the cloaca. [Illustration: FIG. 22.—A mesh of the branchial sac of _Ascidia_, seen A, from inside; B, in horizontal section. _c.d_, Connecting duct; _h.m_, horizontal membrane; _i.l_, internal longitudinal bars; _l.v_, interstigmatic vessels; _p_, _p′_, papillae; _sg_, stigmata; _tr_, transverse vessels.]