Internal Morphology of Some Species of the Genus Rhagasostoma Koschinsky (Cheilostomata, Bryozoa) from a Campanian Erratic Block of Belarus

The internal structure of three species of the genus Rhagasostoma Koschinsky (family Onychocellidae)— R. inelegans (Lonsdale), R. gibbosum (Marsson), and R. rowei (Brydone), originating from the Campanian erratic block of Belarus, was studied. It was found that, in general, all three species have a similar internal structure: the formation of new lineal series of autozooids initiated from the avicularia, the cystids of the zooids narrow towards the basal walls, and the transverse walls of the zooids are located at an angle to the basal walls. The species R. inelegans and R. gibbosum , in contrast to R. rowei , have muscle scars in the distolateral part of the basal wall of autozooids. In the transverse walls of autozooids, R. inelegans and R. gibbosum have one large septulum, while R. rowei has a pair of small septula enclosed in a pore chamber. The comparison of the studied onychocellid species with the type species of the genus Tobolocella Koromyslova et al., as well as with the species of the genus Acoscinopleura Voigt (family Coscinopleuridae) revealed significant differences in their internal structure.