THE SUBCHORDAL CELLS OF OIKOPLEURA-DIOICA AND O-ALBICANS (APPENDICULARIA, CHORDATA)

Oikopleura dioica has two large subchordal cells which were studied in vivo and with light and electron microscopy. They have fixed positions within the haemocoel of the tail but change their morphology continually by protruding and withdrawing processes in an amoeboid manner. Also the fine structure varies considerably from one animal to the next. The cell surface sometimes indicates a strong pinocytotic activity with many coated pits and vesicles. In other cases there are many small vesicles which are interpreted as exocytotic. They are found both in the cytoplasm close to the plasma membrane and in the haemocoelic fluid. Subchordal cells with no pronounced surface activity have a large amount of rough endoplasmic reticulum, which suggests that they synthesize proteins. The presence of subchordal cells in the tail is correlated with the presence of the ontogenetically related bioluminescent oral gland cells in the pharynx; either both types exist simultaneously or both are lacking. It is speculated that the two cell types are also functionally co-ordinated. Oikopleura albicans has a multitude of tiny subchordal cells which have essentially the same fine structure as the two large Oikopleura dioica cells.