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The ultrastructure of the tegument of encapsulated tetrathyridia of the genus Mesocestoides Vaillant, 1863 (Cestoda, Cyclophyllidea, Mesocestoididae) from the liver of root voles Microtus oeconomus (Pallas, 1776) and the structure of the three-layered capsule surrounding them were studied for the first time. Several types of extracellular structures were noted on the surface of the tetrathyridia tegument: vesicles, fine granular material, and vacuoles. In addition, the phenomenon of shedding microtriches, which have expanded parts, was found. Host cells in contact with extracellular material show signs of destruction. A characteristic feature of the capsules surrounding the tetrathyridia is the reticular structure of the fibrous layer containing both native and degenerating inflammatory cells., (© 2023. Pleiades Publishing, Ltd.)

Morphological features of R a u s c h i ta e n ia a n c o ra (Mamaev, 1959), a cestode with atypical mode of attachment («anchoring» in the wall of the host intestine), are considered. At I he center of the overgrown scolex there is a rostellum, size of which is close to that of the developed metacestode. Large suckers are composed mainly of loose parenchyma and fine radial muscle fibers. Fragments of host tissue are noted in the suckers’ cavity. Surface of I he scolex is covered with large microtriches, which are in contact with the host tissue having signs of degradation. Distal cytoplasm is filled with vesicles coming from tegumental cytons. Rostellar glands that are common to cyclophyllids are not found. Excretory canals contain fluid (closer to the surface) and numerous lipid droplets (deeper located canals).

Two cysticercoids, belonging to ascocercus type, namely euascocercus and multicercus, were found in haemocoele of dragonfly larvae of the genus Aeshna from the lakes of the Magadan Province. The cysticercoid of Schistotaenia srivastavai Raush, 1970 (euascocercus) is formed of the outer (exocyst) and inner (endocyst) envelopes, containing the scolex and larval strobila. The outer and inner surfaces of the exocyst are represented by the tegument covered with microvilli. The microvilli of the outer tegument are restricted by the surface layer, consisting of granular and fibrillar material, and possess different structures at different stages of post-embryonic development. The multicercus of Mircia shigini (Konyaev et Gulyaev, 2006) is able to multiply asexually by the endogenous budding. The daughters' individuals are formed in the envelope of the multicercus that represents the tegument bearing microvilli. These microvilli are also restricted by the surface layer. The morphology and development of each individual cysticercoid of the multicercus is similar to those of euascocercus. The production of a great amount of cysticercoids, and the presence of the surface layer resembling the laminated layer of Echinococcus, relates multicercus to hydatid cysts.