Secondary sensory cells in Oikopleura albicans. (A-C) Scanning electron microscopy. The mouth (A) is furnished with a dorsal (dl) and a ventral (vl) lip bearing the circumoral ring (arrowheads pointing to cilia). Cilia are of different lengths and, in the ventral lip (B), are accompanied by short microvilli (mv). In this species, the circumoral ring is not continuous, but interrupted at the lateral edges; in (C), left limit of the upper row of sensory cells (arrow).