Fig. 15.—Ascidia mentula Linn. from the right side (natural size), Loch Fyne, N.B.; Br, Branchial aperture; At, atrial aperture. Arrows show the direction of the water currents.
The Test.—The test is notable amongst animal structures for containing "tunicine," a substance which appears to be identical in composition, and in behaviour under treatment with various reagents, with cellulose. It is cartilaginous in appearance and consistency, and to some extent in structure, as it consists of a clear (or in some cases fibrillated) matrix in which are embedded many corpuscles or cells. It is the matrix that contains the cellulose, which may form over sixty per cent by weight of the entire test. As the test is morphologically a cuticle, being a secretion on the outer surface of the ectoderm (Fig. 16, ec), the cells it contains have immigrated to it from the body, and it has recently been shown that many of these are mesodermal cells (leucocytes or connective tissue wandering cells, amoebocytes, and in some cases embryonic "kalymmocytes," or egg-follicle cells, see below, p. [56]), which have passed through the ectoderm. This process commences in the larval state with the migration of mesenchyme cells from the blastocoele through the epiblast. Ectoderm cells, and possibly also some primitive endoderm cells, also take part in forming the test. Many of these cells in the test remain small and simple, as the fusiform and stellate test-cells; some become pigment-cells, while others enlarge and become vacuolated to form the large (up to 0.15 mm. in diameter) vesicular or "bladder" cells—this is especially the case in the outer layer of the test in Ascidia mentula (see Fig. 17, bl) where there are innumerable clear vesicles, each surrounded by a thin film of protoplasm and having the nucleus still visible at one point of the surface. In some of the Tunicata the test-cells produce calcareous spicules of various shapes (see below, p. p. [86]).
Fig. 16.—Diagrammatic section through test and mantle of Ascidia to show the relations of ectoderm to body-wall and cuticle. bl.c, Bladder-cells; bl.s, blood-sinus; c.t.c, connective tissue cells; ec, ectoderm; mes.c, wandering mesoblast cells; m.f, muscle fibres; t.c, test-cells; t.v, "vessel" of the test."
The test also becomes organised by the growth into it of the so-called "vessels." These are outgrowths of the body-wall covered by ectoderm and containing prolongations of blood-channels from the connective tissue of the "mantle" (body-wall). Fig. 16, t.v shows such an outgrowth, and exhibits the general relations of test (cuticle), ectoderm, and mesoderm. It also explains how it is that the blood-channel being pushed out as a loop gives rise to the double or paired "vessels" seen branching through the test (see Fig. 17, v). The two vessels of a pair are one blood-channel imperfectly divided by a connective-tissue septum. The blood courses out along one side, round the communication in a "terminal knob" at the end, and back down the other side. The "terminal knobs" are very numerous, and form a marked feature in the outer layer of the test (Fig. 17, t.k); in some cases (Culeolus murrayi), they probably form an accessory organ of respiration, while in others (Botryllidae), they pulsate and aid in keeping up the circulation.
The ectoderm is a simple epithelial layer (Fig. 16, ec). It is turned in for a short distance at the branchial aperture (mouth), and atrial aperture (cloaca), as a short stomodaeum and proctodaeum respectively, lined in each case by a delicate prolongation of the test.
Fig. 17.—Section through the surface layer of test of Ascidia mentula, × 50. bl, Bladder cells; t.c, test cell; t.k, terminal knobs of vessels; v, vessels of test.
Fig. 24, A, p. [52], shows the relations of ectoderm, mesoderm, and endoderm in a section through the antero-dorsal part of the body. The cavity marked p.br is a portion of the atrial cavity lined by ectoderm, and must not be confounded with a coelom. The absence of a true coelom in the mesoderm will be noticed in this and other figures, and yet the Tunicata are Coelomata, although it is very doubtful whether the enterocoel which has been described in the development of some is ever found. The coelom is in any case largely suppressed later, and is only represented in the adult by the pericardium and by small cavities in the renal and reproductive organs and ducts.