The branchial aperture or mouth leads into the simple branchial sac or pharynx (Fig. 30, br.s). There are no tentacles. The endostyle is short, is a closed tube both anteriorly and posteriorly (Fig. 29), and has about four longitudinal rows of gland-cells. There is no dorsal lamina, and the peripharyngeal bands run dorsally and posteriorly to unite close in front of the oesophageal opening. The wall of the branchial sac does not show the complex structure usual in Tunicata, and has only two ciliated apertures (Figs. 30, 31, 32, sg). These are homologous with the primary stigmata of the typical Ascidians, and with a pair of the gill-clefts of Vertebrates. They are placed far back on the ventral surface, one on each side of the middle line, and lead into short funnel-shaped tubes which open on the surface of the body behind the anus (Fig. 30, at). These tubes correspond to the right and left atrial involutions, which in an ordinary Ascidian fuse to form the peribranchial cavity. The remainder of the alimentary canal consists of oesophagus, stomach (which may have a glandular diverticulum), intestine and rectum (Fig. 30). The heart, surrounded ventrally by a delicate pericardial membrane, lies below and in front of the stomach, and is formed by the differentiation of the outer ends of epithelial cells into muscular fibrillae. Two specially large glandular cells are placed at the opposite ends of the heart. There are no blood-vessels except the remains of the primary body-cavity (blastocoel). No heart can be seen in some of the smaller species of Oikopleura. Nearly all the species are hermaphrodite, and the large ovary and testis are placed at the posterior end of the body. There is no proper oviduct, the genital products merely breaking through to the exterior at the point marked g.d in Fig. 30. The spermatozoa are generally matured and shed before the ova, and thus self-fertilisation is prevented. The ova are very small, and little is known of the development.
Fig. 30.—Longitudinal optical section of Oikopleura. Part of the tail is cut off. a, Anus; at, atrial opening; br.s, branchial sac; c.f, ciliated funnel; ec, ectoderm; end, endostyle; ep.p, epipharyngeal ridge; g.d, opening of gonads to exterior; ht, heart; hy.p, hypopharyngeal ridge; i, intestine; m, mouth; mus, muscle-bands in tail; n, nerve-cord; n′, nerve in tail; n.ch, urochord; n.g, nerve-ganglion; n.g′, ganglion in tail; oes, oesophagus; or.gl, oral gland; ot, otocyst; ov, ovary; sg, stigmata; so, sense-organ; sp, testis; st, stomach; t, test. (After Herdman.)
Classification.—There are two Families of Larvacea: First, the Kowalevskiidae, including only the remarkable genus Kowalevskia, Fol, in which the heart and endostyle are absent, and the branchial sac is provided with four rows of ciliated tooth-like processes. The two known species have been found in the Mediterranean and in the Atlantic.
The second family Appendiculariidae comprises about eight genera, amongst which may be mentioned:—(1) Oikopleura, Mertens, and (2) Appendicularia, Fol, in both of which the body is short (1 or 2 mm. in length) and compact (Fig. 30), and the tail relatively long, while the endostyle is straight. (3) Megalocercus, Chun, from deep water in the Mediterranean; M. abyssorum is the largest Appendicularian known, having a total length of 3 cm.—it is of a bright red colour. (4) Fritillaria, Q. and G., in which the body is elongated (Fig. 32) and composed of anterior and posterior regions, the tail relatively short, the endostyle recurved, the stigmata opening far in front of the anus, and an ectodermal hood is formed over the front of the body.
In all nearly forty species of Larvacea are known.
Fig. 31.—Transverse section of body and tail of Oikopleura flabellum (?) at, Atrial tube; bl.s, blood-space; br.s, cavity of pharynx or branchial sac; ec, ectoderm; en, endoderm; ep.p, epipharyngeal ciliated bands; gel, gelatinous layer between ectoderm and endoderm; hy.p, hypopharyngeal ciliated band; mus, muscular tissue on inner surface of ectoderm of tail; n, nerve-cord; n′, its continuation in the tail; n.ch, notochord in tail; r, rectum; sg, one of the stigmata or ciliated openings from the branchial sac to the atrial tube; t, test (= young "house"); x, bridge of gelatinous tissue in front of stigma closing branchial sac off from atrial tube. (After Herdman.)
Occurrence.—Although for the most part transparent, and usually almost invisible in sea-water, some Appendicularians may have certain parts of the body (alimentary canal, endostyle, gonads, etc.) brilliantly pigmented (orange, violet, etc.), and may under exceptional circumstances be present in such profusion as to colour tracts of the sea. Appendicularians are widely distributed, having been found in all seas from the Arctic to the Antarctic, both round coasts and in the open ocean. Although a few species have been found at considerable depths in the Mediterranean, still in the Atlantic they are not deep-water animals, and as a group must be regarded as surface-forms. They are fairly abundant to a depth of 100 fathoms, and some few reach 1500. Species of Oikopleura and Fritillaria are frequent round the British coasts, our commonest species being probably O. dioica, Fol, and F. furcata, Moss. Young specimens appear in the plankton about February and March, and larger forms are as a rule found later in the summer. Several instances have been recorded of swarms of especially large forms, provided with massive tests (the "house"), having appeared suddenly on our coast in such abundance as to form an important element in the surface life of the sea.
