Page:EB1911 - Volume 27.djvu/397

Tunicata in the sea than any other factor. It is probable that the occasional phenomenal swarms of Doliolum which have-been met with in summer in the North Atlantic are a result of the curious life-history which, in favourable circumstances, allows a small number of budding forms to produce from the numerous minute buds an enormous number of the next generation. The great increase in the number of species known from nearly all seas during the last twelve or fifteen years of the 19th century enables us now to form a truer estimate of the geographical distribution of the group than was possible when the “Challenger” collections were described, and shows that the Tunicata at least give no support to the “bi-polar theory” of the distribution of animals.

Anatomy

Fig. 1.-Ascidia mentula, from the right side.

at, Atrial aperture; br, branchial aperture; t, test.

As a type of the Tunicata, Ascidia mentula, one of the larger species of the simple Ascidians, may be taken. This species is External Characters. found in most of the European seas, in shallow water. It has an irregularly ovate form, of a dull grey colour, and is attached to some foreign object by one end (fig. 1). The opposite end of the body has a terminal opening surrounded by eight rounded lobes. This is the mouth or branchial aperture, and, it indicates the anterior end of the animal. About half-way back from the anterior end is the atrial or cloacal aperture, surrounded by six lobes and placed upon the dorsal edge. When the Ascidian is living and undisturbed, water is being constantly drawn in through the branchial aperture and passed out through the atrial. If coloured particles be placed in the water near the apertures, they are seen to be sucked into the body through the branchial aperture, and after a short time some of them are ejected with considerable force through the atrial aperture. The current of water passing, in is for respiratory purposes, and it also conveys food into the animal. The atrial current is mainly the water which has been used in respiration, but it also contains all excretions from the body, and at times the ova and spermatozoa or the embryos.

(From Herdman, “Challenger” Report.)
Fig. 2.—Diagrammatic section of part of Mantle and Test of an Ascidian, showing the formation of a vessel and the structure of the test.
m,	Mantle.		blc,	Bladder cell.		mc,	Mantle cells.
e,	Ectoderm.		s, s′,	Blood sinus in mantle being drawn out into test.		y,	Septum of vessel.
tc,	Test cell.
tm,	Matrix.

(From The Cambridge Natural History, vol. vii., “Fishes, &c.” By permission of Macmillan & Co., Ltd.)

Fig. 3.—Section through the surface layer of Test of Ascidia mentula.

bl, Bladder cells; tc, test cell; tk, terminal knobs of vessels; v, vessels of test.

The outer grey part of the body, which is attached at or near its The Test. posterior end and penetrated by the two apertures, is the “test.” This is a firm gelatinous cuticular secretion upon the outer surface of the ectoderm, which is a layer of flat cells. Although at first produced as a cuticle, the test soon becomes organized by the migration into it of cells derived from the mesoderm. A. Kowalevsky has shown that cells of the mesenchyme of the larva make their way through the ectoderm to the exterior during the metamorphosis, and become the first cells of the young test. Some of the cells in the adult test may, however, be ectodermal in origin (see fig. 2). These test cells may remain as rounded or fusiform or stellate cells embedded in the gelatinous matrix, to which they are constantly adding by secretions on their surfaces; or they may develop vacuoles which become larger and fuse so that each cell has an ovate clear cavity (a bladder cell), surrounded by a delicate film of protoplasm with the nucleus still visible at one point; or they may form pigment granules in the protoplasm; or, lastly, they may deposit carbonate of lime, so that one or several of them together produce a calcareous spicule in the test. Only the unmodified test cells and the bladder cells are found in Ascidia mentula (fig. 3). Calcareous spicules are found chiefly in the Didemnidae amongst compound Ascidians; but pigmented cells may occur in the test of almost all groups of Tunicata. The matrix in which these structures are embedded is usually clear and apparently homogeneous; but in some cases it becomes finely fibrillated, especially in the family Cynthiidae. It is this matrix which contains tunicine. At one point on the left side near the posterior end a tube enters the test, and then splits up into a number of branches, which extend in all directions and finally terminate in rounded enlargements or bulbs, situated chiefly in the outer layer of the test. These tubes are known as the “vessels” of the test, and they contain blood. Each vessel is bounded by a layer of ectoderm cells lined by connective tissue (fig. 4, B), and is divided into two tubes by a septum of connective tissue. The septum does not extend into the terminal bulb, and consequently the two tubes communicate at their ends (fig. 4, A). The vessels are formed by an outgrowth of a blood sinus (derived originally from the blastocoele of the embryo) from the body wall (mantle) into the test, the wall of the sinus being formed by connective tissue and pushing out a covering of ectoderm in front of it (fig. 2, s′). The test is turned inwards at the branchial and atrial apertures to line two funnel-like tubes—the branchial siphon leading to the branchial sac, and the atrial siphon leading to the atrial or peribranchial cavity.

Fig. 4.
A,	A vessel from the test.
B,	Diagrammatic transverse section of a vessel.
ec,	Ectoderm.
c t,	Connective tissue.
s, s′,	The two tubes.
y,	Septum.
t k,	Terminal bulb.

The body wall, inside the test and the ectoderm, is formed of a layer (the somatic layer of mesoderm) of connective tissue, enclosing muscle fibres, blood sinuses, and nerves. This layer (the mantle) has very much the shape of the test outside it, but at the two Mantle, Body Wall and Body Cavities. apertures it is drawn out to form the branchial and atrial siphons (fig. 5). In the walls of these siphons the muscle fibres form powerful circular bands, the sphincter muscles. Throughout the rest of the mantle the bands of muscle fibres form a rude irregular network. They are numerous on the right side of the body, and almost totally absent on the left. The muscles are all formed of, very long usiform non-striped fibres. The connective tissue of the mantle is chiefly a clear gelatinous matrix, containing cells of various shapes; it is frequently pigmented, giving brilliant red or yellow colours to the body, and is penetrated by numerous lacunae, in which the blood flows. Inside the mantle, in all parts of the body, except along the ventral edge, there is a cavity—the atrial or peribranchial cavity—which opens to the exterior by the atrial aperture. This cavity is lined by a layer of cells derived originally from the ectoderm^[1]

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1. According to E. van Beneden and Julin (30) only the outer wall of the atrium is lined with epiblast, the inner wall being derived from the hypoblast of the primitive branchial sac.