Morphological redescription of two endocommensal ciliates, Entorhipidium fukuii Uyemura, 1934 and Madsenia indomita (Madsen, 1931) Kahl, 1934 from digestive tracts of sea urchins of the Yellow Sea, China (Ciliophora; Scuticociliatida).

Definitions of the genera Entorhipidium and Madsenia have been updated on the basis of the results of studies on the living morphology and infraciliature of the endocommensal ciliates, Entorhipidium fukuii Uyemura, 1934 and Madsenia indomita (Madsen, 1931) Kahl, 1934, isolated from digestive tracts of the sea urchin, Hemicentrotus pulcherrimus. Entorhipidium are flattened marine endocommensal scuticociliates with a sigmoid body shape and conspicuous tail; the buccal cavity is in the anterior half of the cell and the buccal ciliature has a Uronema-like pattern; the somatic kineties form both apical and post-oral sutures. Because of this new definition, a new combination is suggested: Entorhipidium caudatum (Poljansky, 1951) nov. comb. (basionym: Cryptochilidium caudatum Poljansky, 1951). Madsenia are flattened, slender-bodied endocommensal scuticociliates, having a buccal cavity in the anterior half of the cell with M1 and M2 fused into a single structure and both M3 and the paroral membrane short; the somatic kineties form an anterior suture; no caudal bristle was observed.