Feeding in pterobranch hemichordates and the evolution of gill slits

Abstract

Pterobranch hemichordates reject heavy inedible material which impinges on the frontal surfaces of their tentacles and intercept light edible particles in the food grooves on the frontal surfaces of the arms of their lophophores by impingement and filtration mechanisms which are similar to those of phoronids, bryozoans, and brachiopods. The arrangement of cilia on the tentacles is also similar in the four lophophorate groups. Laterofrontal cells with cilia surrounded by rings of microvilli may detect heavy particles. Low concentrations of heavy particles stimulate secretion from the frontal glands of the tentacles of mucus which binds the particles into strands which are less likely to be dislodged by the water currents. Excess water propelled towards the mouth with the edible material by the cilia on the food-collecting surfaces is allowed to escape from the cleft between the oral lamellae and from the slit between the lamellae and the proboscis of Rhabdopleura. It is suggested that the gill slits of Cephalodiscus may have been formed by the fusion of oral lamellae to allow more powerful currents of water to bypass the mouth. This interpretation of the evolution of gill slits supports previous speculations on the relationships of lophophorates, hemichordates, and chordates.