The polychaetous families ORBINIIDAE, APISTOBRANCHIDAE, PARAONIDAE and LONGOSOMIDAE are reviewed. The ORBINIIDAE are distinguished for 74 species in nine genera and two subgenera. Two new subfamilies, ORBINIINAE and PROTOARICIINAE, are named. Califia, new genus, is erected for C. calida, new species, from California. Other new species are Haploscoloplos bifurcatus, Scoloplos (Leodamas) dendrobranchus and S. (L.) fimbriates, from South Australia, Naineris uncinata from California, and Naineris grubei australis, new subspecies, from South Australia.
The APISTOBRANCHIDAE are known for three species in three genera, and known only from the north Atlantic and Arctic oceans.
The PARAONIDAE are recognized for 28 species in three genera and three subgenera. Aedicira is a new subgenus in Aricidea. New species are Aricidea (Cirrophorus) aciculata, A. (C.) furcata, Paraonis multibranchiata, and P. gracilis oculata, new subspecies, from California.
LONGOSOMIDAE is known for a single species from California.
Aricidea fauveli, new name for A. fragilis, sensu Fauvel, and perhaps A. jeffreysii, sensu Fauvel, originates in the Mediterranean Sea. Haploscoloplos alaskensis is newly referred to H. panamensis, Aricidea heteroseta to A. suecica, A. longicornuta to A. uschakovi and Paraonis filiformis to P. gracilis.
A glossary of terms is given for ORBINIIDAE, and some species are emended.