Fine structure of the ciliated cells and ciliary rootlets of Intoshia variabili (Orthonectida)

Abstract.

The body of the Intoshia variabili (Orthonectida) is covered by a layer of alternating rings of flattened ciliated and non-ciliated cells, covered by a thin 0.25- to 0.30-µm cuticular layer. Ciliated cells bear rows of cilia, with five to seven cilia in each row. The kinetosome of each cilium is situated in a small pit and bears two identical cross-striated rootlets oriented along the longitudinal axis of the body and parallel to the cell surface. Two types of cell contacts are present between these cells, spot desmosomes and tight junctions. Two fibrous bands, an anterior and a posterior one, both running parallel to the cell surface, are always present in the apical part of both the ciliated and the non-ciliated cells. These fibrous bands are not only attached to the desmosomes between the cells within a ring but also connect them, traversing the cell. Thus, two fibrous rings are formed within every ring of ciliated and non-ciliated cells. The rootlets of the cilia closest to the fibrous band are connected with the bands. These bands of filaments and the associated rootlets form a unique structure for the orthonectids. The phylogenetic relationships of Orthonectida are considered on the basis of this new information. The hypothesis that Orthonectida are derived Annelida or at least derived Trochozoa has gained support based mainly on the new ultrastructural data, such as the true cuticle and fibrous bands in the epithelial cells consisting of ciliated and non-ciliated cells.