Each individual of a chain differs from the solitary individual in shape, arrangement of muscle bands, etc., but especially in having reproductive organs. The chain Salpid is hermaphrodite; the embryo develops into a solitary asexual _Salpa_ which produces the chains by budding. The wonderful life history of _Salpa_ was discovered by the poet Chamisso during a voyage round the world in 1819. He observes: “A _Salpa_ mother is not like its daughter or its own mother, but resembles its sister, its granddaughter, and its grandmother.” Here we have an example of “alternation of generations,” a sexual generation (chain form) giving rise to an asexual generation (solitary form), which latter produces the sexual generation.[23] Most of the species of _Salpa_ have double names owing to the chain and solitary forms having been regarded as distinct species before they were known to be phases in the life history of one and the same species. _Salpa runcinata-fusiformis_, solitary form (Fig. 22 B), is barrel-shaped, truncated at each end, with terminal orifices, and with nine muscle-bands on the dorsal surface, some of which converge towards each other. An individual of a chain (Fig. 22 A) is fusiform, with six muscle-bands, and with the orifices not terminal, but at each end of the dorsal surface. The solitary form of _S. africana-maxima_ is barrel-shaped, with truncated ends and terminal orifices, and with nine broad parallel muscle-bands. The chain form is conical at one end, with six bands, and with orifices on the dorsal surface. The exhibited specimen of the chain form, which is in an early stage of growth, contains 202 individuals. The solitary and chain individuals of _Salpa costata-tilesii_ attain a length of six to eight inches. The solitary form has eighteen muscle-bands and two large spines at the posterior end. The individual of the chain has five muscle-bands. A chain of three individuals is exhibited. _Salpa pinnata_ produces a circular chain; the exhibited specimen of the solitary form shows a small chain about to be detached; a circular chain of six individuals is also exhibited. Species of _Salpa_ abound in all seas, but specimens from Naples have alone been exhibited on account of their good preservation. Family _Octacnemidæ_ includes _O. bythius_, a deep-sea Salpid, in which the body forms a flattened disk produced into eight radiating lobes. [Illustration: Fig. 22. _Salpa runcinata-fusiformis_. A. Chain form. B. Solitary form. 1–9, muscle bands; _em_, embryo; _m_, mantle; _visc_, visceral mass or nucleus. (Herdman: _Tunicata_, Encyclopædia Britannica.) ] [Illustration: Fig. 23. _Doliolum denticulatum_, sexual generation, from the left side. _m^1_-_m^8_ muscle bands; _at_, atrial; _br_, branchial apertures; _br s_, branchial sac; _sg_, stigmata; _st_, stomach; _ng_, nerve ganglion; _so_, sense organs. (After Herdman, Encyclopædia Britannica.) ]