An investigation of the feeding behavior of the acorn worm Harrimania planktophilus suggests a novel form of enteropneust feeding with significant phylogenetic implications. H. planktophilus is a holoinfaunal worm that feeds on deposited sediments, and filter feeds on suspended particles in interstitial pore water. To visualize the particle retention behavior involved in filter feeding, adult animals were held in chilled seawater under low light and fed food coloring and fluorescent particles. The behavior was recorded by videography. Most particles ingested were drawn into the mouth by an incurrent flow created by cilia on the pharyngeal bars and without the aid of mucus. Particles that passed freely through the gill pores averaged 3.04 microm, whereas particles retained in the gut and defecated in the feces averaged 13.9 microm. Food coloring entered the mouth and was pumped through the pharynx at a rate of 0.5-2.0 mm/s. There is no evidence of an endostyle or mucus-net capture mechanism in H. planktophilus, but instead particles are filtered and manipulated by a dense covering of cilia on the pharyngeal gill bars. This study suggests that the filter-feeding pharynx is not an innovation of the chordates, but evolved prior to the evolutionary divergence of the hemichordate-echinoderm clade from the chordates.