The solitary Salpa above described is asexual. In the ventral region of its body it forms a stolon which becomes segmented into a series of buds (Fig. 21). As the stolon grows the end series of buds breaks off in the form of a chain and swims away, other chains being detached in succession. A chain is formed of individuals arranged in two rows, the individuals in each row being alternate (not opposite).
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.
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.)
Fig. 23.
Doliolum denticulatum, sexual generation, from the left side. m1-m8 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.)