Fig. 57.—Ascidiozooid of Pyrosoma from the right side. a, Anus; At, atrial aperture; at.m, atrial muscles; Br, branchial aperture; br.s, branchial sac; cl, cloaca; d.l, dorsal lamina; d.t, dorsal tubercle; ec, ectoderm; en, endoderm; end, endostyle; Ht, heart; l.o, luminous organ; mes, mass of mesoderm cells; m.f, muscle fibre; n.g, nerve-ganglion; oes, oesophagus; sg, stigmata; st, stomach; stol, stolon; t, test; t′, projection of test near branchial aperture; tes, testis; tn, tentacle; 1, 2, 3, buds.
Anatomy.—The more important points in the structure of the ascidiozooid of Pyrosoma are shown in Fig. 57. A circle of tentacles, of which one, placed ventrally (tn), is larger than the rest, is found just inside the circular branchial aperture. From this point a wide cavity, with a few circularly placed muscle-bands running round its walls, leads back to the large branchial sac (br.s.), which occupies the greater part of the body. The large stigmata are elongated transversely (dorso-ventrally), and are crossed by internal longitudinal bars running antero-posteriorly. The dorsal lamina is represented by a series of eight or ten languets. The nerve-ganglion (on which is placed a small pigmented sense-organ, the unpaired "eye"), the neural gland, the dorsal tubercle, the peripharyngeal bands and the endostyle are placed in the usual positions. On each side of the anterior end of the branchial sac, close to the peripharyngeal bands is a mass of rounded mesodermal gland-cells (l.o), which are the source of the phosphorescence. They are apparently modified leucocytes lying in blood-sinuses. The alimentary canal is placed posteriorly to the branchial sac, and the anus opens into a large peribranchial or atrial cavity, of which only the median posterior part (cl), is shown in Fig. 57. The heart (Ht) lies between the posterior end of the branchial sac and the intestine, close to where the endostyle is prolonged outwards to form the inner tube of the ventral stolon. The reproductive organs are developed from a cord of germinal tissue which forms a part of every budding stolon, and so establishes a continuity of origin between the ova of successive generations of Pyrosoma. On the ventral edge of the body, immediately behind the stolon, with part of which it is continuous, a portion of this germinal tissue gives rise to a lobed testis (tes), and to a single ovum surrounded by indifferent or follicle-cells.
Development and Life-History.—The development takes place within the body of the parent, in a part of the peribranchial cavity. It is a "direct" development, the tailed larval stage being omitted. The segmentation is incomplete or "meroblastic," and an elongated embryo is formed on the surface of a mass of food-yolk. Follicle-cells, or kalymmocytes, migrate into the embryo, where they aid in its nutrition. The embryo (or young oozooid),[[104]] after the formation of an alimentary cavity, a tubular nervous system, and a pair of laterally placed atrial tubes, divides into an anterior and a posterior part (see Fig. 58). The anterior and ventral part, or stolon, then segments into four pieces (the tetrazooids or first blastozooids),[[104]] which afterwards develop into the first ascidiozooids of the colony, while the posterior part remains in a rudimentary condition, and is what was called by Huxley the "cyathozooid" (Fig. 58, cy). This is really the degenerate oozooid, and eventually atrophies without having completed its development, but having precociously given rise to the budding stolon.
As the four ascidiozooids increase in size, they grow round the cyathozooid and soon encircle it (Fig. 58, B). In this condition the young colony leaves the body of the parent and becomes free. The cyathozooid absorbs the nourishing yolk upon which it lies, and distributes it to the ascidiozooids by means of a heart and system of vessels which have been meanwhile formed. When the cyathozooid atrophies and is absorbed, its original atrial aperture remains and deepens to become the central cavity[[105]] of the young colony, which now consists of four ascidiozooids placed in a ring, around where the cyathozooid was, and enveloped in a common test. The test is at first formed by the ectoderm cells of the cyathozooid. Later it becomes invaded by mesoblast cells from the ascidiozooids in the usual manner. The colony gradually increases by the formation of buds from these four original ascidiozooids. The young colony is, in some species, at first male, and only becomes hermaphrodite when it has attained to some size.
Fig. 58.—Development of Pyrosoma colony. A, young stage showing oozooid or cyathozooid, cy, with stolon divided into four blastozooids (I.-IV.): v, vitellus. B, older stage showing the four blastozooids in a ring around the remains of the cyathozooid. (After Salensky.)
Occurrence.—The half-dozen known species of Pyrosoma are widely distributed over the great oceans, although they are probably most abundant in tropical waters. Pyrosoma atlanticum, Péron, and P. giganteum, Lesueur, are the commonest forms. Although sometimes abundant in the Mediterranean and the North Atlantic they have apparently not been found in British seas. P. elegans, Lesueur, is a Mediterranean form allied to the last two; and P. minatum and P. aherniosum, Seeliger, were discovered during the German "Plankton" expedition in the tropical Atlantic. Finally, the enormous P. spinosum, Herdman, was found by the "Challenger" in both North and South Atlantic in 1873; and some years later (Perrier's P. excelsior) by the French "Talisman" expedition in the tropical Atlantic. The late Professor Moseley said of this ("Challenger") species, "I wrote my name with my finger on the surface of the giant Pyrosoma as it lay on deck in a tub at night, and my name came out in a few seconds in letters of fire." Bonnier and Pérez have recently recorded that they saw an enormous profusion of a large Pyrosoma (up to four metres in length) in the Arabian part of the Indian Ocean.
Order III. Thaliacea (Salpians).
Free-swimming pelagic forms of moderate size, which may be either simple or compound, and in which the adult is never provided with a tail or notochord. Consequently the whole body here corresponds to the trunk only of the Appendicularian without the tail. The test is permanent, and may be either well developed or very slight. In all cases it is clear and transparent. The musculature of the body-wall is in the form of more or less complete circular bands, by the contraction of which water is ejected from the body, and so locomotion is effected. The branchial sac has either two large, or many small, stigmata, leading to a single peribranchial cavity, into which the anus also opens. Blastogenesis takes place from a ventral, endostylar stolon. Alternation of generations occurs in the life-history, and may be complicated by polymorphism. The Order Thaliacea comprises two groups, Cyclomyaria (such as Doliolum) and Hemimyaria (such as Salpa).
Sub-Order 1. Cyclomyaria.