ABSTRACT
The spatangoid Echinocardium cordatum regularly harbours protozoan parasites belonging to the genus Lithocystis (Sporozoa, Gregarinia). Different developmental stages, from early trophozoites to ripe sporocysts, may occur within the body cavity. The host’s coelomocytes react to the presence of motile stages (i.e. trophozoites) of Lithocystis. Trophozoites may be attacked by coelomocytes that progressively surround them. Parasite-associated coelomocytes show a conspicuous transformation, i.e. each cell bears a single spinelike extension so that the whole parasite looks like a minute pincushion. It is suggested that the occurrence of sharp-pointed coelomocytes around trophozoites could be a way to prevent gregarine syzygy and, thus, cyst formation. Trophozoites walled off by such coelomocytes will supposedly degenerate. Encysted stages of Lithocystis (viz. gametocysts and sporocysts) are often found embedded in brown bodies. Brown bodies are particular mesothelial, or mesenteral, covered formations occurring in the echinoid body cavity. The presence of cysts within these bodies is supposedly the consequence of their accumulation on mesenteries. Due to the weight of the cysts, the mesenteries are believed to break down eventually and to lead to the formation of the cystcontaining brown bodies.
