Functional and evolutionary implications of opposed bands, big mouths, andextensive oral ciliation in larval opheliids and echiurids (Annelida)

Larvae of two annelids, the opheliid Armandia brevis and the echiurid Urech is caupo, captured small particles between opposed prototrochal and metatro chal ciliary bands and also captured large particles with wide ciliated mou ths. The body volume of larval A. brevis increased more rapidly than the es timated maximum clearance rate as segments were added. Capture of larger pa rticles by late-stage larvae may compensate for this potentially unfavorabl e allometry. The existence of larvae that use two feeding mechanisms at onc e, not previously known in annelids, suggests possible evolutionary routes between larval forms that feed only with opposed bands (e.g., serpulids and oweniids) and those that use complex oral ciliature to feed primarily on l arge particles (e.g., polynoids and nephtyids). In particular, the metatroc h and food groove of opposed-band feeders may have arisen as expansions of oral ciliation in ancestral large-particle feeders; alternatively, extensiv e oral ciliation in large-particle feeders may have originated as a modific ation of metatroch and food-groove cilia in ancestral opposed-band feeders.