ULTRASTRUCTURE OF THE NERVE-CELLS AND SENSILLA OF GEOCENTROPHORA-BALTICA (PLATYHELMINTHES, LECITHOEPITHELIATA) AND THE SURFACE SENSILLA IN THE GEOCENTROPHORA GROUP

Two types of nerve cell could be distinguished ultrastructurally in th e central nervous system of Geocentrophora baltica (Prorhynchida, Leci thoepitheliata). Both show invaginations in the plasma membrane, but t hey 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 t he presynaptic membrane connected to paramembranous densities. In G. b altica 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 a berrant axoneme and a reduced rootlet; 3. a rootlet branching into man y striated bundles; 4. a thick rootlet; 5. a reduced rootlet and numer ous neurotubules; and 6. collared sensilla each with one cilium in a d eep pit surrounded by a collar of 11 to 12 microvilli. The variable nu mber of microvilli in the collared sensillum is considered plesiomorph ic relative to the stable number of eight microvilli known in sensilla of the Prolecithophora, Proseriata, and Rhabdocoela. The ultrastructu re of the collar sensillum indicates that the Lecithoepitheliata is on ly distantly related to the Prolecithophora and higher turbellarians.