Ultrastructure of trophozoites of the gregarine Lankesteria ascidiae (Apicomplexa : Eugregarinida) parasitic in the ascidian Ciona intestinalis (Protochordata)

The gregarine Lankesteria ascidiae (Lankester 1872) Mingazzini, 1891 was st udied within specimens of the ascidian Ciona intestinalis L. collected from the Bay of Naples. Examinations of parasitized ascidiae by light microscop y showed trophozoites inserted in the host's stomach epithelium or free in the lumen. The gregarine induced an hypertrophic reaction in the host's epi thelium cells, and trophozoites free in the stomach were surrounded by cili a of gastric cells. The host cells flanking infected cells appeared lateral ly compressed and a niche formed as an invagination of the gastric wall cov ering the trophozoite. Transmission electron microscopy of C. intestinalis gastric epithelium showed mononucleated trophozoites with large mitochondri a frequently arranged in peripheral clusters. Maturing trophozoites showed a mucron filled by a dense fibre matrix; these fibres extended in a root-li ke formation through the whole trophozoite cell up to its periphery and app eared to occupy a separate cytoplasmic compartment enclosed in a membrane. The membrane covering the compact mucron was in continuous close contact wi th the membrane of the host cell. The trophozoite's cytoplasm was rich in p araglycogen-like inclusions and electron-dense lipid deposits. The surface showed rows of electron-dark knobs protruding at regular intervals from a b asal layer. These rows were cross-connected by underlying perpendicular bun dles of filaments.