III ASCIDIAE COMPOSITAE——DIDEMNIDAE, ETC. o7 
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 
Fic. 50.—Calcareous spicules of the Tunicata, enlarged. A, From Cystodytes ; B, from 
Leptoclinum; C, from Culeolus ; D, from Rhabdocynthia. 
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. 
Fam. 4. Diplosomatidae.—Test reduced in amount (Fig. 51), 
rarely containing spicules. Vas deferens not spirally coiled. In 
Fic. 51.—Section of a colony of Diplosoma (enlarged) to show the small amount of test 
present. Or, Branchial aperture ; c.cl, common cloaca ; ¢, test. 
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. 
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 
