Post-embryonic development and fully-formed polycephalic larvae of Taenia parva Baer, 1926 were examined by light (LM) and transmission electron microscopy (TEM). Three developmental stages were recognised: (1) an early stage of exogenous budding at the surface of the central vesicle; (2) a stage of polycephalic cyst development accompanied by segmentation of the growing larval strobile and an obvious decrease in the size of the central vesicle; (3) fully-formed larval strobile and invaginated scoleces. In fully-developed encysted polycephalic larvae, there are usually 14-24 segmented larval strobilae, each terminating with an invaginated scolex; larval strobilae arise from a common central vesicle and remain attached posterior to it during the entire development. The number of segments varies between 109 and 120 per larval strobila. The polycephalic larvae examined closely resemble the strobilocercus type of taeniid larvae. The structure of developing and fully-formed larvae was examined by TEM. The tegument, scolex, subtegumental musculature of the strobilar segments, protonephridial system, calcareous corpuscles and medullary parenchyma of larvae exhibit general similarity with the same structures in adults at both LM and TEM levels. The morphogenesis of the larva of T. parva is compared with that of the polycephalic larvae of other Taenia spp. (T. krepkogorski, T. twitchelli and T. endothoracica) and with other asexually-multiplying cestode larvae (mesocestoidids, hymenolepidids and dilepidids).
Ultrastructural features of juvenile cestodes (metacestodes) can provide useful characters for phylogenetic and evolutionary analyses. Until now, however, they have been relatively little utilised (Beveridge 2001, Chervy 2002). The postembryonic development and structure of fully formed metacestodes were examined in two cyclophyllideans: Taenia parva Baer, 1926 (Taeniidae); and Sobolevitaenia verulamii (Mettrick, 1958) Korniushin, 1972 (Dilepididae). In T. parva, three developmental stages were recognized: (1) an early stage of exogenous budding at the surface of the central vesicle; (2) a stage of polycephalic cyst development accompanied by segmentation of the growing metacestode strobila and an obvious decrease in the size of the central vesicle; (3) a fully formed metacestode of the strobilocercus type with 14–24 invaginated scoleces. The tegument, scolex, subtegumental musculature of the strobilar segments, protonephridial system, calcareous corpuscles and medullary parenchyma of larvae exhibit general similarity to the same structures in adults at both LM and TEM levels. The morphogenesis of the metacestode of T. parva is compared with that of polycephalic metacestodes of other Taenia spp. (T. krepkogorski, T. twitchelli and T. endothoracica) and with other asexually multiplying metacestodes (Mesocestoides vogae, hymenolepidids and dilepidids). In S. verulamii, the body of the cysticercoid with invaginated scolex armed with a double crown of rostellar hooks was completely surrounded by the cercomer, which appears to be separated from the cyst and scolex. The surface of the suckers is covered with a thick layer of glycocalyx. Five cell types were distinguished in the sections: (1) perikarya of metacestode tegument; (2) glycolgen-storing parenchymal cells; (3) glandular-type cells with large, electron-dense secretory-like granules; (4) flame cells; and (5) calcareous corpuscle-forming cells. The surface of the cercomer is covered by elongated microvilli, which evidently differ from characteristic microtriches covering all other parts of the metacestode surface. The ultrastructure of S. verulamii evidently differs from that of the other dilepidid cestode examined to date, Lateriporus geographicus, the cyst wall of which more resembles cysticercoids of Hymenolepididae than those of Dilepididae. Concluding remarks: Ultrastructural studies on metacestodes have considerable promise for providing important new characters for phylogenetic analysis. New TEM data on a great variety of cestode species are urgently needed. Until now, this field has not been exploited in a systematic fashion. Until more comprehensive studies become available, the current data are not yet amenable to analysis.
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