AbstractThe ultrastructure of the central nervous system of four species of theGeocentrophoragroup (Prorhynchida, Lecithoepitheliata) has been studied withGeocentrophora waginias a model organism. This is the first ultrastructural study of the nervous system in flatworms of the endemicGeocentrophoragroup from Lake Baikal.The neurons are characterized by invaginations of the plasma membrane with extracellular material (ECM) extending deep into the neuronal cytoplasm. Two types of neuron are distinguished on the basis of the character of the cytoplasm and the content of neuronal vesicles. Several more vesicle types are observed in the nerve processes in the neuropile.The following kinds of neuronal release sites are distinguished: single synapses, shared synapses, nonsynaptic release sites characterized by omega profiles and neuromuscular contacts. Special features of the synapses in the species ofGeocentrophoraare pronounced local thickenings of the presynaptic membrane connected to paramembraneous densities. The ECM‐filled invaginations and the local presynaptic thickenings are features that distinguish the neurons ofGeocentrophoraspp. from previously described turbellarian neuron