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 {103}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. [Illustration: 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.)] [Illustration: 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.)]