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). 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.
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.
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 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.
