Ciliary Feeding Assisted by Suction From the Muscular Oral Hood of Phoronid Larvae.

Phoronid larvae have an oral hood upstream from a postoral encircling array of ciliated tentacles. Cilia move water over the hood and between the tentacles, from anterior to posterior. When an algal cell or other particle in this current contacts a tentacle, the neighboring part of the hood lifts, and the particle is drawn toward the mouth. The correlation between movements of hood and particles indicates that the particle moves with water entering the enlarged space beneath the hood. Each lift of the hood is preceded by contact between a particle and a tentacle. A hood lift follows contact with a particle anywhere along the length of a tentacle, and clearance rates are thus proportional to the total length of tentacles deployed and the velocity of the current past the tentacles. After being detained at the ciliary bands of tentacles, particles are transported by the hood lift at speeds exceeding measured transport along the frontal ciliated surfaces of other larval forms. Faster transport may aid capture of faster prey. The larva's feeding mechanism is unique to the phylum Phoronida. Larvae of brachiopods, bryozoans, hemichordates, and echinoderms have similar ciliary bands producing feeding currents, but none are known to transport food toward the mouth by suction produced by muscle contractions.