ASCIDIAE COMPOSITAE DIDEMNIDAE, ETC. 



87 



Testis single, large ; vas deferens spirally coiled (Fig. 49). 

 The chief genera are — Bidemnum, 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 



Fig. 50. — Calcareous spicules of the Tunicata, enlarged. A, From Cystodytes ; B, from 

 Leptodinum; 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. Yas deferens not spirally coiled. In 



c.cl. 



Fig. 51. — Section of a colony of Diplosoma (enlarged) to show the small amount of test 

 present, hr, Branchial aperture ; c.cl, common cloaca ; t, 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-h&As, 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. miraUle (Fig. 44, C), the 



