In the family Ascidiidae the eggs are minute and contain little or no food-yolk, and the tailed larvae (Figs. 26, 42, A) are of the typical form and structure described in Chapter II.
Fam. 3. Cynthiidae.—Solitary fixed Ascidians (Fig. 39), sometimes occurring in aggregations, but never forming colonies; usually with leathery or fibrous, opaque test, which is sometimes encrusted with sand; branchial and atrial apertures usually both four-lobed. Branchial sac longitudinally folded (Fig. 36, A); stigmata straight; tentacles simple or compound (Fig. 37); neural gland dorsal to ganglion; gonads attached to body-wall. This family is divided into three sections:—
Fig. 36.—Diagrammatic transverse sections of branchial sacs of Cynthiidae. A, Cynthia; B, Styela; C, Styelopsis; D, Pelonaia. Br.f 1-7, First to seventh branchial fold; d.l, dorsal lamina; end, endostyle; mh, meshes.
Sub-Fam. 1. Styelinae.—Not more than four folds (Fig. 36, B) on each side of branchial sac; tentacles simple (Fig. 37, A). The more important genera are—Styela, Macleay, and Polycarpa, Heller (Fig. 39), with stigmata normal; and Bathyoncus, Herdman, with stigmata absent or modified. There are a very large number of species of both Styela and Polycarpa from all parts of the world, including our own seas. A very abundant British littoral form has been placed in an allied genus under the name Styelopsis grossularia (Fig. 39, A). It is known in some places round our coasts as "the red-currant squirter." This species has only one well-marked fold in the branchial sac (Fig. 36, C). Another exceptional British Styelid is Pelonaia corrugata, Forb. and Goods. (Fig. 39, I), with no branchial folds (Fig. 36, D).
Sub-Fam. 2. Cynthiinae.—More than eight folds in branchial sac (Fig. 36, A); tentacles compound (Fig. 37, B); body sessile or with a short stalk (Fig. 39, F). The chief genus is Cynthia, Savigny, with a large number of species, some of which are British. Rhabdocynthia has echinated calcareous spicules in the mantle (see Fig. 50, D, p. 87).
Forbesella tessellata is a remarkable British species, having the test marked out into plates (Fig. 39, B). It is intermediate in some characters between Styelinae and Cynthiinae.
Fig. 37.—Tentacles of Cynthiidae. A, Simple, in Styelinae; B, Compound, in Cynthiinae.
