Summary

On the evolution of the stomatogastric nervous system in Plathelminthes

While all euplathelminthes disposing of a pharynx have a pharyngal nervous system comprising one or two nerve rings as well as a highly autonomous nerve plexus linked to the central nervous system, the knowledge on the intestinal part of the stomatogastric system cannot be considered fully satisfactory yet – except for findings established in triclades and trematodes. On the basis of refined alizarine techniques the stomatogastric nervous system in various groups of turbellaria was successfully demonstrated in a way that makes it obvious that most turbellaria have an autonomous nervous system consisting of a pharyngal and an intestinal part, which gives rise to a number of new aspects regarding the evolution of this organic system within the group of spirals. An intestinal nerve plexus of an irregular and reticular structure without differentiating pronounced tracks was demonstrated in catenulides, prorhynchides, graffillides, protoplanellides as well as all seriata, and the autonomous function of the intestinal peristalsis was proved in the course of experiments. The lacking intestinal nervous system in numerous typhloplanides and all acoeles has to be interpreted as a retrograde evolution in the wake of the retromorphosis of an intestinal musculature of its own, or as an evolution of a syncytial central parenchyma. Thus it can be said that the existence of an intestinal nerve plexeus represents a characteristic feature of spirals. The natural relationship among all spirals proved by the spiral‐like cleavage as well as corresponding cell lineage is substantiated once more by the evidence of an autonomously working intestinal nerve plexus in plathelminthes.