The inner structures of the cerebral vesicle in larvae of the ascidian, Styela plicata, were examined by scanning electron microscopy. The ocellus of this species is situated in the postero-dorsal wall of the cerebral vesicle. The ocellus is as degenerate as that of the other species of Styela which consists of only a single cell containing a small pigment body. The cell body of the statocyte was found to be attached to the ventral floor of the cerebral vesicle by two junctions. The cell body is hollowed between the junctions, forming an inverted pigment cup. Four types of protuberance were recognized in the cerebral vesicle. Three of these project from the ventral floor under the cell body of the statocyte. The distribution of these three types is restricted to the floor on the right side of the junctions of the statocyte. The fourth type of protuberance was always found as a pair on the left part of the postero-dorsal wall of the cerebral vesicle. Probable functions of these protuberances are discussed.
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