1. Populations of a previously undescribed polychaete, Aglaophamus neotenus, were found in three polyhaline locations on the Maine coast. Specimens were also received from Canadian workers. Temperatures in the Maine locations ranged from 0° C in winter to 17° C in the summer.
2. Diagnostic characters of the adult include heavily ciliated, involute interramal cirri which bear accessory cirri, present from setiger 5, sometimes 6, through setigers 15 to 20; the presence of acicula with recurved tips, capillary tipped, barred and spinous setae, and the lack of furcate setae; two pairs of antennae on the prostomium; ornamentation of the proboscis; the lack of eversible digitiform nucal processes; and the presence of pygidial eyespots on a spherical pygidium which is ringed with red pigment.
3. Adults reproduced at a size smaller than was previously reported in the Nephtyidae. Spawning was observed in the laboratory. Sperm are primitive. The eggs are granular, white in color, and resemble human erythrocytes in shape. Eggs average 70 µ in widest diameter.
4. Clacuteeavage was spiral and appeared similar to that of other polychaetes.
5. Larval stages included: protrochophore, early trochophore, trochophore, Metatrochophore I, Metatrochophore II, and an eight-setiger swim-crawl-stage. Metamorphosis of the swim-crawl stage resulted in a ten-setiger juvenile which was essentially adult in morphology.
6. Behavior of adults and larvae was observed. Adults burrowed actively and swarm above the sediment even when undisturbed. Larvae swam vigorously in early stages but slowed down as the swim-crawl stage was approached. Juveniles burrowed actively through the sediment and made mucus-lined burrows.
7. The diet of A. neotenus was studied. Larvae are lecithotrophic through the trochophore stage but diatoms were observed in the gut of Metatrochophore I stages. Observation of gut contents indicated juveniles feed on sediment. The digestive tract of adults contained materials, revealed by the contents of fecal pellets, which demonstrated that they were nonselective omnivores, feeding on sediment.