Fig. 20.—Transverse section of the endostyle of Ascidia mentula, × 350. bl.s, Blood-sinus; end.l, lips of the endostyle; gl, glandular tracts; i.l, internal longitudinal bar; l.v, interstigmatic vessels; m, mantle; p.br, peribranchial cavity; sg, stigmata; v.bl.s, ventral blood-sinus.
This organ corresponds to the hypopharyngeal groove of Amphioxus and the median part of the thyroid gland of Vertebrata. It is interesting to notice that the (at least) four longitudinal tracts of gland-cells are of remarkable constancy, being found not only in all groups of Tunicata, including even the pelagic, tailed Appendicularians, but also in Amphioxus and in the young thyroid gland of the Ammocoete. When, in Ascidians, a third marginal glandular tract is added it has a different appearance from the two characteristic tracts. The mucus is carried forward by the action of the large floor-cilia of the endostyle (Fig. 20) to the groove between the peripharyngeal bands, and after encircling the anterior end of the branchial sac and collecting the food particles, it passes backwards along the dorsal edge of the branchial sac to the oesophagus, guided by a membranous fold, the dorsal lamina (Fig. 21, d.l), which is more or less ridged or corrugated, and may be armed with marginal tags or even replaced by larger processes (the "languets") in some species of Ascidians. In the living animal the lamina has its free edge curved to the right hand side in such a manner as to constitute a fairly perfect tube along which the train of food passes.
Fig. 21.—Antero-dorsal part of pharynx in Ascidia mentula, × 15. br.s, Part of branchial sac; d.l, dorsal lamina; d.t, dorsal tubercle; p.br.z, prebranchial zone; p.p, peripharyngeal bands; sph, sphincter of branchial aperture; tn, tentacle.
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 pharyngo-cloacal slits) placed close to the dorsal lamina and leading direct to the cloaca.
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.
Fig. 22 shows a small part of the wall of the branchial sac, in which it may be seen that the bars containing the blood-channels are arranged in three regular series:—(1) The "transverse vessels" which run horizontally round the wall and open at their dorsal and ventral ends into large median longitudinally running tubes, the dorsal blood-sinus (or "dorsal aorta") behind the dorsal lamina, and the ventral blood-sinus (or "branchial aorta") beneath the endostyle; (2) the fine longitudinal or "interstigmatic vessels" which run vertically between adjacent transverse vessels and open into them, and which therefore 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 (Fig. 22, i.l) communicate with the transverse vessels by short side branches where they cross, and at these points are prolonged into the cavity of the sac in the form of hollow papillae. In some Ascidians (e.g. Corella and most of the Molgulidae) the interstigmatic vessels are curved so that the stigmata form more or less complete spirals (see Figs. 35 and 41). In some species of Ascidia, and other Ascidians, the interstigmatic vessels are inserted into the transverse vessel in an undulating course in place of the straight line seen in Fig. 22, B, l.v, the result being that the stigmatic part of the wall of the branchial sac seems to be folded or thrown into microscopic crests and troughs. This is known as "minute plication." In some cases, again (Cynthiidae), the whole wall of the sac is pushed inwards at intervals to form large folds visible to the eye (see Fig. 36, A and B). The intersections of the internal longitudinal bars with the transverse vessels divide up the inner surface of the branchial sac wall into rectangular areas called "meshes." One such mesh, containing eight stigmata in a row, is seen in Fig. 22, A. The internal longitudinal bars bear papillae at the angles of the meshes, and occasionally in intermediate positions. There are frequently horizontal membranes (Fig. 22, B, h.m) attached to the transverse vessels between the papillae. There are many "connectives" running from the outer wall of the branchial sac to the mantle outside, and allowing the blood in the transverse vessels to communicate with that in the sinuses of the mantle (see Fig. 19, con).
Heart and Circulation.—It is one of the notable features of the Tunicata that the circulation is not constant in direction, but is periodically reversed.
The blood of Ascidians is in the main transparent, but usually contains certain pigmented corpuscles in addition to many ordinary leucocytes or colourless amoeboid cells. The pigment in the coloured cells may be red, yellow, brown, or in some cases blue or opaque white. The blood may reach the branchial sac either from the dorsal or from the ventral median sinus according to the direction in which the heart is beating at the moment (see below); and it is a most interesting and beautiful sight to see the circulation of the variously coloured corpuscles through the transparent vessels, and the lashing of the cilia along the edges of the neighbouring stigmata in a small Ascidian under the microscope.