Placida viridis is a little known Mediterranean ascoglossan that has not been examined in detail since 1893. The redescription includes new observations on the reproductive system, central nervous system, and radula; it points out that the animal lacks a heart. We hope it will help collectors who find P. viridis, or a related species, on the coasts of other continents to come more surely to a decision. The evidence presented indicates that Placida (type species, Placida viridis Trinchese, 1873) can no longer be regarded as a subgenus of the genus Hermaea [type species, Hermaea bifida (Montagu, 1844)]. It is recommended that the genus Hermaea be transferred to a restricted family Hermaeidae and the genus Placida remain in the family Stiligeridae.