| Abstract: | The nervous system of three species of the Temnocephalida has been studied using the GAIF method (which in small flatworms mostly reveals sensory catecholaminergic neurons). These species represent the evolution from the primitive “turbellarian-like” temnocephalids to the most specialised ones with tentacles and a sucker. The numbers and positions of GAIF-positive neurons are invariant within each species and do not change from hatching to full maturity. A characteristic unpaired neuron contributing to the innervation of the anterior margin of the body is present in all species: such a cell has previously been found only in marine Thalassovortex tyrrhenicus (Dalyellidae) which confirms close relationships between these taxa. Our series of species shows (i) a reduction in number of GAIF-positive perikarya associated with the lateral cords and reduction of GAIF-positive innervation on the ventral side of the body, which is probably related to the loss of ciliary locomotion (the shift to passive hunting and looping locomotion) and (ii) reinforcement of the GAIF-positive innervation of the anterior end of the body which begins to play an important role in capturing the prey. The retention of the medial unpaired neuron and nearly identical sets of GAIF-positive neurons in Diceratocephala boschmai and Craspedella pedum (rather different in morphology) give the first indication (in the Plathelminthes) of persistence of homologous neurons through significant evolutionary transformations of the organs they innervate. |
