[Illustration: FIG. 49.—Section of _Leptoclinum_ colony, showing the distribution of spicules and parts of the ascidiozooids. _b_, Base of colony; _br_, branchial aperture; _br.s_, branchial sac; _sp_, spicules; _st_, stomach; _tes_, testis; _v.d_, vas deferens.] FAM. 3. DIDEMNIDAE.—Colony usually thin and incrusting. Test containing stellate calcareous spicules (Figs. 49 and 50, B). {87}Testis single, large; vas deferens spirally coiled (Fig. 49). The chief genera are—_Didemnum_, Savigny, in which the colony is thick and fleshy, and there are only three rows of stigmata on each side of the branchial sac; and _Leptoclinum_, Milne-Edwards, in which the colony is thin and incrusting (Fig. 49), and there are four rows of stigmata. Colonies of _Leptoclinum_, forming thin white, grey, or yellow crusts under stones at low water, are amongst the commonest of British Compound Ascidians. [Illustration: FIG. 50.—Calcareous spicules of the Tunicata, enlarged. A, From _Cystodytes_; B, from _Leptoclinum_; C, from _Culeolus_; D, from _Rhabdocynthia_.] FAM. 4. DIPLOSOMATIDAE.—Test reduced in amount (Fig. 51), rarely containing spicules. Vas deferens not spirally coiled. In _Diplosoma_, Macdonald, and other allied genera (Fig. 51), the larva is gemmiparous (Fig. 42, F). Some species are common British forms, especially on _Zostera_-beds and amongst seaweeds. [Illustration: FIG. 51.—Section of a colony of _Diplosoma_ (enlarged) to show the small amount of test present. _br_, Branchial aperture; _c.cl_, common cloaca; _t_, test.] FAM. 5. POLYCLINIDAE.—Ascidiozooids divided into three regions—thorax, abdomen, and post-abdomen (Fig. 46, C). Testes numerous; vas deferens not spirally coiled. The chief genera are—_Pharyngodictyon_, Herdman, with stigmata absent or modified, containing one species, _Ph. mirabile_ (Fig. 44, C), the {88}only Compound Ascidian known from a depth of 1000 fathoms; _Polyclinum_, Savigny, with a smooth-walled stomach (Fig. 52, A); _Aplidium_, Savigny, with the stomach-wall longitudinally folded (Fig. 52, B); _Morchellium_, Giard, with an "areolated" stomach (Fig. 52, D), bearing knobs on the outside; and _Amaroucium_, Milne-Edwards, in which the ascidiozooid has a long post-abdomen and a large atrial languet, and where the stomach-wall shows longitudinal ridges breaking up into knobs (pseudo-areolated, Fig. 52, C). The last four genera contain many common British species. [Illustration: FIG. 52.—Various conditions of stomach in Polyclinidae. A, _Polyclinum molle_, Herdman; B, _Aplidium zostericola_, Giard; C, _Amaroucium proliferum_, M.-Edw.; D, _Morchellium argus_, M.-Edw.] Many of the Compound Ascidians die down in winter; but amongst Polyclinidae, as in _Clavelina_, a form of hibernation is found, the old ascidiozooids dying, but some of the buds in the basal part of the colony accumulating a large store of reserve-material in their ectoderm, and lying dormant until spring, when they regenerate the colony.