ULTRASTRUCTURE OF THE CILIARY PITS IN THE GEOCENTROPHORA GROUP (PLATYHELMINTHES, LECITHOEPITHELIATA)

The ultrastructure of the paired lateral ciliary pits in several endem ic species of Geocentrophora from Lake Baikal and in one cosmopolitan species, G. baltica, has been compared and the possible functional sig nificance is discussed. The pit is composed of two distinctive parts; the bottom of the pit is an extensive sensitive area, filled with uni- and biciliary sensory receptors with reduced rootlets and numerous neu rotubules. The walls of the pit are formed by several large 'dark cell s', characterized by a dark cytoplasm with numerous mitochondria, a la rge nucleus, intracellular canaliculi, basal infoldings of the cell me mbrane, glycogen granules and a varying number of cilia. A protruding, densely ciliated ridge occurs along the anterior wall of the pit. The cilia have a strengthened rootlet system and seem to provide a strong water current into the pit. Dark cell processes penetrate the basemen t membrane of the pit and come into the vicinity of large cells with a cytoplasm similar to that of the 'dark cells' of the pit. These large cells in their turn come close to the terminal parts of the protoneph ridial canals, containing a weir. Smaller protonephridial capillaries without a weir seem to open directly into the pit lumen. The morpholog ical data obtained suggest that the ciliary pit in not only a sensory structure, but plays a part in osmoregulation and ion exchange as well .