Fig. 61.—Salpa runcinata-fusiformis. A, aggregated or "chain" form; B, solitary form. At, Atrial aperture; at.m, atrial muscles; Br, branchial aperture; br.m, branchial muscles; d.l, dorsal lamina or "gill"; d.t, dorsal tubercle; emb, embryo; end, endostyle; m, mantle; m.b, muscle-bands; n.g, nerve-ganglion; p.p, peripharyngeal bands; st, stolon; st″, "chain" of buds; t, test; v, visceral "nucleus."
Fig. 62.—Diagram to show the arrangement and connexion of the aggregated zooids in a young chain of Salps. 1, 3, 5, zooids on the right; 2, 4, 6, zooids on the left. At, Atrial aperture of a zooid; Br, branchial aperture of another; c.t at the top of the figure points to three pairs of connecting tubes; c.t at the foot, to two pairs. Each zooid is united to each of the four neighbours it touches by a pair of connecting tubes, and so has eight such tubes in all.
Occurrence and Reproduction.—The family Salpidae[[108]] includes the single genus Salpa, Forskål, which, however, may be divided into two well-marked groups of species—(1) those such as S. (Cyclosalpa) pinnata, in which the alimentary canal is stretched out ("ortho-enteric" condition) along the ventral surface of the body, and (2) those such as S. runcinata-fusiformis, in which the alimentary canal forms a compact globular mass (Fig. 61, v), the "nucleus" ("caryo-enteric" condition), near the posterior end of the body. About fifteen species altogether are known; they are all pelagic in habit, and are found in nearly all seas. Each species occurs in two forms (Fig. 61, A and B), the solitary asexual (proles solitaria), and the aggregated sexual (proles gregaria), which are in most species quite unlike one another, the aggregated form being usually more rounded, ovoid, or fusiform (Fig. 61, A), and the solitary more quadrangular, and often provided with conical processes or projecting points.
Fig. 63.—Diagram to show the relations of the groups of young buds, when first formed on the stolon of Salpa. at, Atrial aperture; br, branchial aperture; el, elaeoblast; end, endostyle; h, heart; n.g, nerve-ganglion; ov, ovum; s, stolon; st, stomach; I, II, III, groups of buds. (After Brooks.)
Fig. 64.—Transverse section through endostyle and young stolon of Salpa pinnata. ec, Ectoderm of parent reflected at ec′ to cover base of stolon; ec″, ectoderm of stolon; end, endoderm of stolon; g, ovary; mes, mesoderm cells; n, nerve-tube of stolon; p.br, peribranchial tubes of stolon. (After Brooks.)
The solitary form gives rise, by gemmation at the posterior end of the endostyle (Fig. 63), to a complex tubular stolon, containing processes from the more important organs of the parent-body, which give rise to an endodermal tube, two peribranchial tubes, a neural tube, two blood-sinuses and mesoblast cells, a genital cord, and over all the ectodermal covering (see Fig. 64). This stolon becomes segmented (Fig. 63) into a series of buds or young "chain" individuals, of which there may be several hundreds. As the stolon elongates (Fig. 61, B, st″), the buds undergo lateral shifting, and rotation round their longitudinal axis, so as to acquire the relations seen in the "chain," which then emerges from the tube in the test through which it has been growing, so as to project to the exterior near the atrial aperture. The buds at its free end which have now become far advanced in their development are set free in groups, which remain attached together by processes of the test, each enclosing a diverticulum from the body-wall (Fig. 62), so as to form "chains." Each member of the chain is a Salpa of the sexual or aggregated form, and when mature may—either still attached to its neighbours or separated from them—produce one or several embryos (Fig. 61, A, emb), which develop into the solitary form of Salpa. Thus the two forms, different in appearance and structure and different in mode of origin, alternate regularly in the life-history of Salpa.