5. The cavities of the renal vesicles and of the gonads and ducts are spaces formed in the mesoblast. They have been variously interpreted:—
(a) As of the same nature as the blood spaces (blastocoelic), or
(b) As formed by a splitting of the mesoblast (coelomic).
6. The cavity of the neural gland and of its duct opening at the dorsal tubercle is derived from the primitive dorsal neural tube of the embryo, and so may be regarded as a part of the lumen of the cerebro-spinal nervous system.
Tentacles, etc.—The branchial aperture leads through the branchial siphon into the branchial sac. At the base of the siphon, just about the line of junction of the ectoderm of the stomodaeum with the endoderm of the mesenteron, is placed a circle of simple hair-like tentacles (Fig. 18, tn) which stand out at right angles to the wall, and more or less completely meet in the centre to form a delicate, sensory grid or sieve through which all the water entering the body has to pass. These tentacles not only act mechanically, but are also sensitive although only scattered sensory cells, and no specially differentiated sense-organs are found upon them. Behind the tentacles lies the plain, or papillated, prebranchial zone (Fig. 21, p.br.z), bounded behind by a pair of parallel and closely placed ciliated ridges with a groove between—the peripharyngeal bands—which encircle the anterior end of the branchial sac.
Fig. 19.—Semi-diagrammatic transverse section of Ascidia, passing through the atrial aperture, seen from anterior surface, left side uppermost. At, Atrial aperture; at.l, atrial lobe; Br.s, branchial sac; cl, cloaca; con, connective; d.bl.s, dorsal blood-sinus; d.l, dorsal lamina; end, endostyle; g.d, genital ducts; i, i′, intestine; l.v, interstigmatic vessel; m, mantle; m.b, muscle-bundles; ov, ovary; p.br, peribranchial cavity; r, rectum; ren, renal vesicles; sg, stigmata; sph, atrial sphincter; t, test; tr, transverse vessel; ty, typhlosole; v.bl.s, ventral blood-sinus.
The branchial sac is very large—much the largest organ of the body—and extends almost to the posterior end of the body, while the rest of the alimentary canal lies upon its left side. The food particles, consisting of microscopic plants and animals, are carried in through the branchial aperture by the current of water, but most of them do not pass out through the gill-slits to the atrium, being entangled in the viscid mucus which passes by ciliary action along the groove between the peripharyngeal bands.
Endostyle.—The mucus just referred to is produced in the long canal-shaped gland called the endostyle or hypobranchial groove, which runs along the entire ventral edge of the branchial sac (Fig. 18, end). The sides, and especially the floor of the endostyle, are richly ciliated, while there are four (or six) strongly-marked, peculiarly-shaped glandular tracts, two (or three) on each side (Fig. 20, gl) running along its length, and separated by areas of closely-packed fusiform cells with short cilia, amongst which are found some bipolar sensory cells.
