"A large group containing both compound and simple ascidians, distinguished chiefly by the character of the branchial sac, which is flat, without large pleats or folds, though minute undulations may occur. A system of slender, tubular, internal longitudinal vessels (usually numerous and equally spaced) raised on supporting papillae is normally present on the inner surface of the sac, though in a few genera it has become rudimentary or lost. Branchial tentacles always simple; stigmata either straight or spiral in arrangement; gonads present on one side of the body only, usually situated in, or in close relation to, the intestinal loop. Test usually more or less gelatinous and translucent, though often tough." (Van Name 1945: 155)
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