Figure 1

Secondary sensory cells in Oikopleura dioica. (A-D) Scanning electron micrographs. (A-B) Juvenile 26 hours old (A) and detail of the mouth (B) to show the cilia of sensory cells belonging to the circumoral ring. Arrowheads: cilia belonging to sensory cells; dl: dorsal lip; t: tail; tr: trunk; vl: ventral lip. (C-D) Mouth of an adult animal showing cilia (white arrowheads) of sensory cells located on the dorsal lip. Cilia are of different lengths; this confers a wavy arrangement to the circumoral ring. Note that the apical membrane of supporting cells (sc) forms a crest delimiting the sensory bundle. The square area in C is enlarged in D to show that each sensory cell possesses a number of ciliary rows that form the sensorial apparatus. (E) Sagittal section of the head showing the ventral (vl) and dorsal (dl) lips and the circumoral ring (black arrowheads); white arrowheads: perypharyngeal band; cd, ciliated duct of the neural gland; e, endostyle; ph, pharynx. (F-K) Transmission electron microscopy of circumoral organ. The organ is formed by a single cell row dorsally located on the roof of the oral cavity and ventrally on the tip of the ventral lip (vl). Squared areas in F are enlarged in G and H to show dorsal and ventral receptor cells, respectively. The hair bundle is multiciliated and delimited by apical extensions of supporting cells (sc). In I cilia show a conventional 9+2 microtubular arrangement. G shows a dense, short basal body (bb) with developed ciliary rootles (cr). Arrowheads: cilia of sensory cells; bl, basal lamina; dl, dorsal lip; G, Golgi complex; n, nucleus; ph, pharynx; tj, tight junction. Note that in J and K neurites (ne) are very close to the sensory cell membrane (sen).