This embryo sexually produced inside the body of an aggregated form becomes a solitary _Salpa_ (such as Fig. 61, B), which differs in appearance, structure, and habits from its parent, and has no reproductive organs. After swimming for a time, however, it develops the ventral stolon on which buds form which are eventually sexual Salpae. These are set free from the solitary form in sets, still connected together, and they may swim about together for a time as a chain of aggregated Salpae before separating to become the adult sexual individuals (such as Fig. 61, A). CLASSIFICATION.—_Salpa_ may be divided into the following subgenera:[110]—_Cyclosalpa_, Blainville, in which the alimentary canal is ortho-enteric, and the "chain" consists of individuals united in a circle; _Iasis_, Savigny, with several embryos formed at a time; and _Pegea_, Sav., _Thalia_, Blumenbach, and _Salpa_, Forskål, all with one embryo only, and differing from one another in the condition of the "gill" and other details: all except _Cyclosalpa_ have the alimentary canal caryo-enteric. _Cyclosalpa_ has three species, the best known of which is _C. pinnata_ of the Mediterranean, a form possessing light-producing organs like those of _Pyrosoma_, but placed along the sides of the body. _Salpa_ has four or five species, one of which, _S. runcinata-fusiformis_ (Fig. 61), has occasionally been found in British seas; _Thalia_ includes the species _T. democratica-mucronata_, which has been sometimes obtained in swarms in the Hebridean seas, or cast ashore on our southern or western coasts; _Pegea_ has the species _P. scutigera-confoederata_; and _Iasis_ contains the remaining half-dozen species, the best known of which is _I. cordiformis-zonaria_, the only other Salpian which has been found in British seas. {109}[Illustration: FIG. 68.—A, solitary form of _Octacnemus bythius_ (after Moseley); B, diagram of structure of _Octacnemus_ (after Herdman); C, aggregated form of _O. patagoniensis_ (after Metcalf). 1, from outside; 2, with test removed; and 3, with mantle removed. _a_, Anus; _adh_, area of attachment; _at_, atrial, and _br_, branchial aperture; _br.s_, branchial sac; _end_, endostyle; _g.s_, gill-slits; _i_, intestine; _n.y_, nerve-ganglion; _oe_, oesophagus; _ov_, ovary; _p.br_, peribranchial cavity; _st_, stomach; _stol_, stolon.]