ULTRASTRUCTURE OF THE EPIDERMIS OF MEARA-STICHOPI (PLATYHELMINTHES, NEMERTODERMATIDA) AND ASSOCIATED EXTRA-EPIDERMAL BACTERIA

The ultrastructure of the epidermis of Meara stichopi Westblad, an endosymbiont in the holothurian Parastichopus tremulus (Gunneri) was studied. The entire body surface of the worm is ciliated. The columnar epithelial cells interdigitate with each other. The epidermal cells are not insunk and have a prominent terminal web underlined by mitochondria. Each cilium has a main, anteriorly pointing, cross-striated rootlet with a knee and a posterior rootlet dividing into two lateral fibre bundles. These bundles descend to contact the two neighbouring rootlets in the row behind. The basal bodies contain glycogen granules. Epidermal glands containing secretion globules are common. Some epidermal glands contain rhabdoids. Between the epidermal cilia we found elongated procaryotic organisms, evidently symbiotic gram-negative bacteria, anchored with a tapering end in the glycocalyx. The observations imply that the ultrastructure of the epidermis of M. stichopi is similar to that described earlier from Nemertoderma and Flagellophora (Nemertodermatida). Thus these three genera share some systematically interesting ultrastructural character states, especially the pattern of the ciliary rootlet system.