On the Larval Protonephridial System of Gyrocotyle and the Evolution of Cercomeromorphae (Platyhelminthes)

Abstract

AbstractEggs of Gyrocotyle urna specimens were hatched. Certain results from the hatching work are given. The protonephridial system of the larvae/lycophores was investigated. The system was found to be bilaterally symmetrical with two nephridiopores in the anterior body‐half. The system is very similar to the posterior protonephridial system in the oncomiracidium (the monogenoidean larva). Unlike the oncomiracidium the Gyrocotyle lycophore has no anterior system. The hooks of amphilinidean and gyrocotylidean lycophores were found to be of the same basic type as those of other cercomero‐morphaean larvae. A preliminary investigation of the hook ontogeny of Amphilina foliaceawas made: all ten hooks appear to begin developing at the same time, the six ordinary hooks early assumed a similar shape and arrangement to those in the oncosphaera, the four aberrant hooks seem to be less aberrant in their earlier developmental stages. On a basis of the number of hooks and other facts relating to the larvae, cercomero‐morphaean evolution at the larval level is claimed. Because there are no traces of a mouth, a pharynx and an intestine in the ontogeny of Gyrocotyle, the lack of these body parts in the lycophore is assumed to be a primitive condition. It is claimed that the monogenoidean protonephridial system originated from a system of the Gyrocotyle lycophore type and that the mouth, pharynx and intestine are secondarily evolved in the monogenoideans. It is assumed that mouth‐, pharynx‐ and intestine‐less creatures with an ingestive body‐covering gave rise to the digenean and the cercomeromorphaean evolutionary lines. On a basis of the ontogeny and the life cycles in the different cercomeromorphaean groups it is claimed that the adult monogenoideans never reached a certain evolutionary level which is repeated in the ontogeny of the ces‐todes, the amphilinideans and the gyrocotylideans.