The morphological adaptations of the alciopid and lopadorhynchine polychaetes for a pelagic mode of life are compared, with particular reference to the organs concerned with feeding and reproduction. Both groups are predatory, but feed in different ways; Lopadorhynchus has poorly-developed eyes but long cirri, anterior segments modified for grasping, and large pharyn-geal glands; alciopids have large eyes and the most advanced have long eversible proboscides. Many alciopids have evolved a copulatory mechanism whereby the sperm are transfered to the female which stores them in receptacula until required. The phylogenetic relationships of the different genera are briefly described.