Fam. 2. Ascidiidae.—Solitary fixed Ascidians, never forming colonies; with gelatinous or cartilaginous test; branchial aperture usually eight-lobed, atrial aperture usually six-lobed; branchial sac not folded; internal longitudinal bars usually present; stigmata straight or curved; tentacles simple; gonads in or around the intestinal loop. This family is divided into three sections:—
Sub-Fam. 1. Hypobythiinae.—Branchial sac with no internal longitudinal bars, test strengthened with curious symmetrically placed nodules.
The one genus Hypobythius, Moseley, contains two stalked deep-water forms found by the "Challenger;" H. calycodes (Fig. 34, A), from the North Pacific, 2900 fathoms, and H. moseleyi from the South Atlantic, 600 fathoms.
Fig. 34.—A, Hypobythius calycodes, Moseley; B, Chelyosoma macleayanum, Brod. and Sowb.; C, Corynascidia suhmi, Herdman; D, Rhodosoma callense, Lac.-Duth.
Sub-Fam. 2. Ascidiinae.—Internal longitudinal bars present; stigmata straight. Many genera, of which the following are the more important:—Ciona, Fleming, dorsal languets present; Ascidia, Linnaeus (in part Phallusia, Savigny), dorsal lamina present (Fig. 15, p. [40]); Rhodosoma, Ehrenberg, anterior part of test modified to form operculum (Fig. 34, D); Abyssascidia, Herdman, intestine on right side of branchial sac. The type genus of this section, Ascidia, has been described in detail above (Chapter II. p. [39]), and Figs. 15 to 26 illustrate its structure and life-history. There are many species. Ciona intestinalis, Linn. (Fig. 40, B), is one of the commonest of British Ascidians, and lives readily in aquaria.
Sub-Fam. 3. Corellinae.—Stigmata curved and forming spirals (Fig. 35). Three genera:—Corella, Alder and Hancock, test gelatinous, body sessile; Corynascidia, Herdman, test gelatinous, body pedunculated (Fig. 34, C), a remarkable deep-sea form with very delicate spirally-coiled vessels in the branchial sac (Fig. 35, A), found in the Pacific (2160 faths.) and the Southern Ocean; Chelyosoma, Brod. and Sowb., upper part of test modified into horny plates (Fig. 34, B).
Fig. 35.—A, branchial sac of Corynascidia suhmi, Herdman; B, branchial sac of Corella japonica, Herdman. i.l, Internal longitudinal bars; tr, transverse vessels. (After Herdman.)
Corella contains several British species, one of which, C. parallelogramma, O. F. Müll., is one of the commonest and most handsome Ascidians in our coralline zone (about 20 faths.). Through its clear crystalline test the lemon-yellow and carmine pigmentation of the mantle, and even (with a lens) the working of the cilia along the spiral stigmata of the branchial sac (compare Fig. 35, B), can readily be seen. The beating of the heart can be seen just in front of the viscera upon the right side of the branchial sac (compare with Ascidia, Fig. 23).