A new species of enteropneust, Harrimania planktophilus, lives intertidally and subtidally in mixed sediments in Barkley Sound, British Columbia, Canada. H. planktophilus has a long proboscis skeleton extending into the pharyngeal region. The collar (mesosome) has complete dorsal and ventral mesenteries. The trunk (metasome) has four distinct regions that can be recognized externally: the branchial region, esophageal region, hepatic region, and an undifferentiated intestinal region leading to the anus. The dorsal pharynx is large and has long gill slits without synapticles. Posterior to the gills is a constriction followed by a short esophageal region and a long gonadal region. The paired dorsolateral gonads extend almost to the end of the trunk. Eggs in the ovaries appear amber yellow, and the testes appear slightly paler. The trunk terminates at an anus with a well-developed sphincter muscle. H. planktophilus forms long sinuous burrows that are semipermanent and shared. Females deposit a tubular egg mass in a burrow in which the embryos develop directly into juveniles. Gastrulation appears to be by invagination, followed by a ciliated stage that has a telotrochal swimming band, suggesting that the ancestor to H. planktophilus developed via a tornaria larva. The juveniles emerge from the egg membrane with a ventral post-anal tail and assume an interstitial burrowing life habit. The post-anal tail, mode of development, small size and correlated simplification in body plan suggest that H. planktophilus is closely related to Saccoglossus, and together these worms may be sister taxa to the colonial Pterobranchia. A taxonomic key is provided to the enteropneust genera, and to the species of Harrimania: