47 



of stigmata on each side of the branchial sac. Colonies of 

 Leptoclinum, forming thin, white, grey, or yellow crusts, 

 under stones at low water, are amongst the connnonest of 

 British Compound Ascidians. 



Family IV. Diplosomid^ : — Test reduced in amount, 

 rarely containing spicules. Vas deferens not spirally 

 coiled. In Diplosoma , the most important genus, the 

 larva is gemmiparous. This is a common British form, 

 especially on Zosfera beds, and amongst sea-weeds. 



Family V. Polyclinid^ : — Ascidiozooids divided into 

 three regions, thorax, abdomen, and post-abdomen. Testes 

 numerous ; vas deferens not spirally coiled. The chief 

 genera are — Pharyngodictyon, with stigmata absent or 

 modified, one species : the only Compound Ascidian 

 known from a depth of 1000 fathoms ; Polyclinum, with 

 a smooth-walled stomach ; Aplidium, with the stomach- 

 wall longitudinally folded; and Amaroucium,m. which the 

 Ascidiozooid has a long post-abdomen and a large atrial 

 languet. The last three genera contain many common 

 British species. 



Family VI. Botryllid.^ : — Ascidiozooids having the 

 intestine and reproductive organs alongside the branchial 

 sac. Dorsal lamina present ; internal longitudinal bars 

 present in branchial sac. The chief genera are — Botryllus, 

 with simple stellate systems, and Botrylloides* with 

 elongated or ramified systems. There are many species 

 of both these genera, which form brilliantly coloured fleshy 

 crusts under stones and on sea-weed at low tide. They are 

 amongst the commonest and the most beautiful of British 

 Ascidians. 



* It is intended that a future L.M.B.C. Memoir will deal with But rijllu ides 

 as a type of the Compound Ascidians. 



