On gemmation of Polycercus paradoxa (Cestoda, Cyclophyllidea) metacestode generations

摘要

The gemmation of P. paradoxa (Rud., 1802) Spasskaja et Spassky, 1970 metacestodes in the infested earthworms Eisenia sp. was studied. The parietal gemmation of lacunar blastogene, the second generation of metacestodes developing gemmae of cycticercuses, as well as the gemmation of rhizoid metacestoda (rhizomorph) were described. In the process of invagination of the blastogene side, a gemma is plunged into the primary lacuna of blastogene which gradually disappears for the morphogenesis of the cystecercus. The similarity of P. paradoxa blastogene to the early developmental stage of Monopylidiidae metacestodes may testify that its morphogenesis stops at the stage of megalosphere. In the P. paradoxa blastogene any body compartments, which are homologous to those of metacestodes, are difficult to be distinguished. This blastogene is the common body of an asexual colony. Thus, polycers is an acercomeric cystocercoid. A cause of simplifying the P. paradoxa blastogene ontogenesis and morphology as well as of rhizomorph appears to be the shift of gemmation of the early stages of developing metacestodes. This fact proves that the heterochrony plays a significant role in evolution of asexual reproduction of animals.