An atrial membrane, similar to that previously reported (as a placental membrane) in Placentela crystallina Redikorzev, 1913, has been found in the holotype and other specimens of another aplousobranch ascidian, Ritterella tokioka Kott, 1992 (=R. pedunculata Tokioka, 1953). In serial sections, the membrane is seen to be an extensive fold of the atrial epithelium over the rectum that projects into the atrial cavity and supports developing embryos over its outer surface. The similar states in the atrial membrane seen in the two species are attributable to homoplasy, rather than homology.
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