This paper reports upon the marine athecate and athecate hydroids collected by the Danish expedition to the Kei Islands (Indonesia) in 1922. Concomitantly, the historic collection of hydroids from the Moluccas made by C. Pictet and M. Bedot in 1890 was revised. The family Tubidendridae Nutting, 1905 is recognized as valid and its diagnosis emended. Taxonomic problems with the genera Campalecium and Mitrocomium are discussed. The new species Cladocarpus keiensis is described. Ectopleura pacifica Thornely, 1900 is considered to be a subjective junior synonym of Tubularia viridis Pictet, 1893. Acryptolaria rectangularis (Jarvis, 1922) is removed from the synonymy of Acryptolaria angulata (Bale, 1914). Gymnangium unjinense Watson, 2000 is recognized as a subjective synonym of Lytocarpia orientalis (Billard, 1908). The following 15 species are new records for Indonesian waters: Hydractinia granulata Hirohito, 1988; Balella mirabilis (Nutting, 1905); Garveia clevelandensis Pennycuik, 1959; Eudendrium kirkpatricki Watson, 1985; Corydendrium corrugatum Nutting, 1905; Cladocoryne haddoni Kirkpatrick, 1890; Solanderia secunda (Inaba, 1892); Campanulina panicula G. O. Sars, 1874; Acryptolaria rectangularis (Jarvis, 1922); Acryptolaria angulata (Bale, 1914); Lafoea dumosa (Fleming, 1820); Zygophylax rufa (Bale, 1884); Salacia punctagonangia Hargitt, 1924; Synthecium flabellum Hargitt, 1924; Antennella campanulaformis (Mulder & Trebilcock, 1909). A lectotype is designated for Synthecium samauense Billard, 1925. Halecium simplex Pictet, 1893 is transferred to Mitrocomium, as M. simplex, n. comb. Thecocarpus perarmatus Billard, 1908 is transferred to Lytocarpia, as L. perarmata, n. comb., and T. myriophyllum var. orientalis, Billard, 1908 is raised to full species level, as Lytocarpia orientalis, n. comb., n. status. Aglaophenia pluma var. sibogae Billard, 1913 is raised to full species level as A. sibogae, n. status.
Descriptors: Cladocarpus keiensis, Lytocarpia orientalis n. comb., Aglaophenia sibogae n. status, Mitrocomium simplex n. comb., Lytocarpia perarmata comb nov.
Tropical West Atlantic
Zoogeography, Biogeography (generalities), Geographic distribution