Abstract
Placentela crystallina REDIKORZEV, 1913 has been noted as one of the remarkable representatives of tunicates showing the formation of placental structures, but its morphology is given rather incompletely and its systematic position has been unsettled. On the other hand, Sigillinaria clavata OKA, 1933 is proved to bear the placental membrane as in P. crystallina. Close examinations of the type specimen of P. crystallina, the specimens of S. clavata identified by OKA and other related specimens made S. clavata sink to a junior synonym of P. crystallina. Structures of the placental membrane, probably functioning as a resting bed for embryos, are presented here in detail. P. crystallina is also unique among the ascidians provided with the postabdomen (containing at least epicardium and gonad), in having the heart situated along the posterior part of intestinal loop. The taxonomic significance of this structure seems to lead to the conclusion that P. crystallina is better placed provisionally in the subfamily Euherdmaniinae of the family Polyclinidae.
