In the present study of the Anactinotrichid order Holothyrida, five species are dealt with: Australothyrus ocellatus nov. gen., nov. spec. (family Allothyridae) from Queensland, Australia; Hammenius (Leiothyrus) holthuisi nov. subgen., nov. spec. and Hammenius (Thonius) montanus nov. spec. (family Holothyridae), both from New Guinea; and Holothyrus coccinella Gervais and Holothyrus legendrei nov. spec. (family Holothyridae), both from Mauritius. The paper includes detailed studies of the morphology, a study of the postembryonic development, a reclassification of the species of the family Holothyridae, and notes on the relationships of the order Holothyrida.