Catinia plana Bocquet and Stock, the type genus and species of the family Catiniidae, which has been known incompletely is redescribed on the basis of type material. Structure of appendages, especially those of mandible and maxilla, are reinterpreted. In addition, two new species of Myzomolgus associated respectively with the sipunculids Sipunculus nudus Linnaeus and Siphonosoma cumanense (Keferstein) are described from Korean waters. A comparison of already known species of Myzomolgus and two new species of the same genus resulted in a conclusion that members of this genus possess homogeneous traits and the genus is a well established taxon. A key to distinguish five genera of the Catiniidae is also provided.