Free-swimming pelagic forms which exhibit alternation of generations in their life-history, and in the sexual condition form colonies. The body is more or less fusiform, with the long axis antero-posterior, and the branchial and atrial apertures nearly terminal and opposite. The test is well developed but transparent. The musculature of the body-wall is in the form of a series of transversely-running bands which do not usually form complete independent rings as in the CYCLOMYARIA. These partially-encircling muscles in the Salpidae (see Fig. 61, _m.b_) are probably to be regarded as modified branchial and atrial sphincters which have spread over the intervening body. The branchial and peribranchial (cloacal) cavities form a continuous space in the interior of the body, opening externally at the ends by the branchial and atrial apertures, and traversed obliquely from the dorsal and anterior to the ventral and posterior end by a long narrow vascular ciliated band, which represents the dorsal lamina, the dorsal blood-sinus, and the neighbouring parts of the dorsal edge of the branchial sac of an ordinary Ascidian. The alimentary canal is placed ventrally. It may either be stretched out so as to extend for some distance anteriorly, or, as is more usual, be concentrated to form along with the testis a rounded opaque mass near the posterior end of the body, known as the visceral mass or "nucleus." The embryonic development is direct, no tailed larva being formed. The embryo is united to the parent for a time by a "placenta." This sub-order contains, in addition to its typical members, the SALPIDAE, another still somewhat problematical family the OCTACNEMIDAE, including a single very remarkable deep-water genus (_Octacnemus_), which in some respects does not conform with the characters given above, and exhibits a certain amount of affinity with the primitive fixed forms from which Salpidae have been derived. {102}[Illustration: 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."] [Illustration: 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.]