Fig. 67.—Young solitary Salpa democratica-mucronata attached to the parent by the placenta. atr.ap, Atrial aperture; br, dorsal lamina; cil.gr, dorsal tubercle; ebl, elaeoblast; end, endostyle; n.gn, nerve-ganglion; oes, oesophagus; or.ap, branchial aperture; peric, pericardium; pl, placenta; rect, intestine; stol, stolon; stom, stomach. (From Parker and Haswell, after Salensky.)
At an early period in the development a part of the surface of the embryo, on its ventral edge, becomes separated off, along with a part of the wall of the cavity ("oviduct"—a diverticulum from the atrium) in which it lies, to form the "placenta" (Fig. 67, pl) in which the embryonic and maternal blood-streams circulate in close proximity, and so allow of the conveyance of nutriment to the developing embryo by means of large migrating placental cells. At a somewhat later stage a number of cells placed at the posterior end of the body alongside the future nucleus become filled up with oil-globules to form a mass of nutrient material—the "elaeoblast" (Fig. 67, ebl)—which is used up later in the development. Many suggestions have been made as to the homology and meaning of the elaeoblast; but it may now be regarded as most probable that it is reserve food-material associated with the disappearing rudiment of the tail found in the larval condition of most Ascidians. The development is direct; and it may be said, then, that this young asexual (solitary) Salpa differs from the corresponding form in the life-history of Doliolum (Fig. 60, A) in that its tail is no longer a locomotory organ, but is represented by a nutritive mass, the elaeoblast, while the body, in place of being free, is attached by its ventral surface to a special organ of nutrition—the "placenta"—in connexion with the blood-stream of the parent.
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.
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.
The family Octacnemidae includes the single remarkable genus Octacnemus, now known in a solitary and an aggregated form. It was found during the "Challenger" expedition, and was first described by Moseley. It is apparently a deep-sea representative of the pelagic Salpidae, and may possibly be fixed at the bottom. The body in the solitary form is somewhat discoid, with its margin prolonged to form eight tapering processes, on to which the muscle-bands of the mantle are continued. The alimentary canal forms a compact nucleus, which is attached to an apparently imperforate membrane which stretches across the body, separating the branchial from the atrial cavities. The endostyle is very short, and the dorsal lamina is also much reduced. The reproduction and life-history are entirely unknown. The aggregated form consists of a small number of individuals united by a slender cord composed of test, body-wall, and endodermal tissue. Octacnemus has been found[[111]] in the South Pacific from depths of 1070 and 2160 fathoms, and off the Patagonian coast from 1050 fathoms. Two species have been described: O. bythius, Moseley, and O. patagoniensis, Metcalf. Metcalf, who has recently investigated the aggregated form (O. patagoniensis), considers that the genus is more nearly related to the Clavelinidae than to the Salpidae. Possibly its position might be best indicated by a line diverging from near the point (3) in the phylogenetic diagram below.
General Conclusions.
The following diagram is a graphic representation of the genetic affinities, or what is now generally supposed to have been the probable course of phylogeny of the Tunicata. It will be noticed that it shows (1) the Proto-Tunicates arising from Proto-Chordata, not far from the ancestors of Amphioxus (see also, this vol. p. [112]); (2) that the Larvacea are regarded as the most primitive section of the group; (3) that the Thaliacea (Doliolidae and Salpidae) are supposed to be derived not directly from primitive pelagic forms, but through the early fixed Ascidians, not far from (4) the ancestral compound Ascidians, which gave rise to the Pyrosomatidae; (5) that the Ascidiidae and other higher Simple Ascidians are derived, like the Compound Ascidians, from ancestral Clavelinidae; and (6), that the Ascidiae Compositae are polyphyletic, the Holosomata (Botryllidae and Polystyelidae) being derived from ancestral Simple Ascidians independently of the Merosomatous families.
