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:— [Illustration: 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). {75}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. [Illustration: FIG. 37.—Tentacles of Cynthiidae. A, Simple, in Styelinae; B, Compound, in Cynthiinae.] [Illustration: FIG. 38.—_Culeolus wyville-thomsoni_, Herdman. A, from left side (half-nat. size); B, part of branchial sac. _At_, Atrial aperture; _Br_, branchial aperture; _br.f_, branchial fold; _i.l_, internal bar; _sp_, spicules; _tr_, transverse vessel. (After Herdman.)]