_a_, branchial; and _b_, atrial orifice; _c_, tunic or test; _d_, mantle; _e_, branchial sac; _f_, gullet; _g_, stomach; _h_, anal orifice; _i_, dorsal lamina; dotted line indicates the endostyle. ] The Ascidian is orientated as follows: hold the animal with the branchial orifice pointing forwards and the atrial upwards; the branchial orifice will be anterior and the opposite end posterior; the atrial orifice will lie on the upper or dorsal aspect, the opposite aspect being lower or ventral, and the sides right and left. The aspects, in fact, correspond with those of a vertebrate animal. A vertical section roughly shows the animal to be formed of three concentric sacs (Figs. 2, 3). The outermost, which is tough and membranous, is called the Test or Tunic, the whole group owing its name to the presence of this protective covering. [Illustration: Fig. 3. Diagrammatic dissection of _A. mentula_. _at_, atrial orifice; _br_, branchial orifice; _a_, anal orifice; _brs_, branchial sac; _dl_, dorsal lamina; _end_, endostyle; _m_, mantle; _ng_, nerve ganglion; _oea_, orifice of gullet; _pbr_, peribranchial cavity; _st_, stomach; _t_, test; _tn_, tentacles. (After Herdman: _Tunicata_, Encyc. Britannica.) ] The middle sac, termed the Mantle, which almost corresponds in shape to the outer, is composed of connective tissue, muscle-fibres, blood-vessels, etc.; in spirit specimens, the mantle is shrunk away from the test except at the orifices and at a point behind, where vessels enter the test. The innermost or Branchial Sac is attached behind the branchial orifice and along the ventral edge, but otherwise hangs free in the interior, the space around and outside of the sac being termed the atrial or peribranchial cavity. The delicate walls of the branchial sac, which resemble fine muslin, are perforated by innumerable vertical slits, termed stigmata, arranged in transverse rows (Fig. 4). [Illustration: Fig. 4. _Ascidia mentula._ Part of wall of branchial sac showing stigmata. Magnified. ] The margins of the stigmata are lined with cilia which set up currents; and the water which enters by the branchial orifice, passes through the stigmata into the atrial cavity, and thence out through the atrial orifice. The walls of the branchial sac are chiefly composed of a sieve-like meshwork of fine blood-vessels arranged in transverse and longitudinal rows. The currents of water passing through the stigmata aërate the blood in the vessels. Besides the stigmata, the branchial sac has two relatively large orifices, viz., the branchial orifice or mouth, and, at the opposite end, the opening into the gullet. The branchial sac is, in fact, a capacious throat or pharynx (Diagram Fig. 2 and Fig. 14). Inside the branchial orifice is a circle of fine tentacles, which guard the entrance to the branchial sac. The food of the animal consists of minute animal and vegetable organisms.