Two types of nerve cell could be distinguished ultrastructurally in the central nervous system of Geocentrophora baltica (Prorhynchida, Lecithoepitheliata) . Both show invaginations in the plasma membrane, but they differ in the character of the cytoplasm (light or densely stained) and the distribution of the neuronal vesicles (evenly or in groups) . Different kinds of vesicles and neuronal release sites are observed . Special features of the synapses are pronounced local thickenings of the presynaptic membrane connected to paramembranous densities . In G. baltica and five endemic Geocentrophora spp . from Lake Baikal six types of surface sensillum were observed at the epidermal surface : 1 . those with a long thin rootlet ; 2 . a short, balloon-shaped cilium with an aberrant axoneme and a reduced rootlet ; 3 . a rootlet branching into many striated bundles ; 4 . a thick rootlet ; 5 . a reduced rootlet and numerous neurotubules ; and 6 . collared sensilla each with one cilium in a deep pit surrounded by a collar of 11 to 12 microvilli . The variable number of microvilli in the collared sensillum is considered plesiomorphic relative to the stable number of eight microvilli known in sensilla of the Prolecithophora, Proseriata, and Rhabdocoela . The ultrastructure of the collar sensillum indicates that the Lecithoepitheliata is only distantly related to the Prolecithophora and higher turbellarians . 183

The ultrastructure of the paired lateral ciliary pits in several endemic species of Geocentrophora from Lake Baikal and in one cosmopolitan species, G. baltica, has been compared and the possible functional significance 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 neurotubules . The walls of the pit are formed by several large `dark cells', characterized by a dark cytoplasm with numerous mitochondria, a large nucleus, intracellular canaliculi, basal infoldings of the cell membrane, 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 basement 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 protonephridial canals, containing a weir. Smaller protonephridial capillaries without a weir seem to open directly into the pit lumen . The morphological data obtained suggest that the ciliary pit in not only a sensory structure, but plays a part in osmoregulation and ion exchange as well .