History of British animals - University of Guam Marine Laboratory

( 467)MOLLUSCA TUNICATA.DICHITONIDA.Inner tunic detached from the external one,and united only atthe two orifices. The branchiae are large, equal, and spreadon the central surface of the inner sac, and the branchial orificehas an inner membranaceous ring,or circle of tentacula.All the British Dichitonida are fixed, and the branchial andanal orifices are not oppositeto each other.A. Body simple.a.Apertures furnished with four rays. The bodyfour on each side, the meshes uninterrupted.sessile; branchiaePandocia.aa. Apertures with indistinct rays, or more than four, the tunic soft ;branchiae destitute of longitudinal folds.b.Body pedunculated.Clavellina.bb.Body sessile.c. Tunic and branchial cavity straight.Pirena.Ciona.cc.Tunic turned up at the base.Phallusia.AA. Body compound.a. Branchial orifice radiated.b. Branchial and anal orifices with six regular rays ; body sessile.Polyzona.bb. Branchial orifice only with six regular rays.aa.c. Body pedunculated ; system single, circular, and terminal.cc.Sydneum.Body sessile, polymorphous.Alpidium.Branchial orifice simple.Botryllus.Gg '"