Bergström, 1914
Small worms with prostomium rounded, without antennae. Two minute palps and two eyes. Proboscis eversible, with or without a pair of lateral chitinous hooks. First two or ten segments of the body with reduced parapodia. Later parapodia uniramous with short dorsal and ventral cirri, a longer setigerous lobe and compound spinigerous setae. Day (1967) mentions that species of the genus Iospilus include some of the few holoplanktonic polychaetes which feed on diatoms, and species of Phalacrophorus which have chitinous hooks are possibly predators. Uschakov (1972) suggested that these jaws are developed as adaptive structures to pelagic life, for catching rapidly moving planktonic organisms.
Two genera:
Genus Iospilus Viguier, 1886
Genus Phalacrophorus Greeff, 1879