Ciliation and function of the food-collecting and waste-rejecting organs of lophophorates

Abstract

Experiments with mixtures of colloidal graphite, stained Sephadex particles, and algae; observations by stroboscopic interference contrast optics; and scanning and transmission electron microscopy suggest that phoronids, brachiopods, and bryozoans can simultaneously reject waste material by an impingement mechanism and accept edible particles by a filtration mechanism without the ciliary reversals suggested in previous models of suspension feeding in lophophorates. Specialized laterofrontal cilia, which may detect heavy inedible particles, are found on the tentacles of all three phyla of lophophorates. In phoronids and bryozoans edible material is carried towards the esophagus by components of water currents created by the lateral cilia of the tentacles of the lophophore while inedible particles are rejected by the frontal cilia of the tentacles. The passage of food material to the mouths of brachiopods is assisted by frontal cilia located in grooves on alternate tentacles while the frontal cilia of ungrooved tentacles reject inedible material. The epistomes of lophophorates are also involved in the simultaneous acceptance of food and rejection of solid waste material and allow the escape of excess water travelling towards the mouth with food particles. This finding of a functional significance for the epistome suggests that lophophorates deserve reassessment as possible ancestors of chordates.