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.

Serial electron microscope reconstructions were used to examine the organization and cell types of the nerve plexus that surrounds the mouth in amphioxus larvae. The plexus is involved in a rejection response that occurs during feeding: a number of oral spines project across the mouth, and debris impinging on them triggers a contraction of the gill slit and pharyngeal musculature that forces water through the mouth, dislodging the debris. The oral spine cells are secondary sense cells that synapse with neurites belonging to a class of peripheral interneurons intrinsic to the oral nerve plexus. These in turn synapse with a second class of peripheral neurons with large axons that we interpret as sensory cells and which probably transmit signals to the nerve cord. The intrinsic cells also appear to synapse with each other, implying that local integrative activities of some complexity occur in the oral plexus. In comparative terms, the intrinsic neurons most closely resemble the Merkel-like accessory cells of vertebrate taste buds, and we postulate a homology between oral spine cells and taste buds despite di¡erences in function. There are also similarities between the amphioxus oral plexus and adoral nerves and ganglia of echinoderm larvae, suggesting homology of both the oral nerve plexus and the mouth itself between lower deuterostome phyla and chordates.

Lacalli, T. C. and Gilmour, T. H. J. 2001. Locomotory and feeding effectors of the tornaria larva of Balanoglossus biminiensis. —Acta Zoologica (Stockholm) 82: 117–126 The tornaria ciliary bands and oesophagus were examined ultrastructurally to identify the neural components that control larval behaviour. The circumoral ciliary band is known to be innervated in part by fibres from the apical plate and adoral nerve centres. Within the band itself, however, the only neurones we could find were multipolar cells, an unusual cell type with apical processes that traverse the surface of the band. Similar cells occur in the circumoral bands of echinoderm larvae. The tornaria telotroch has a much larger nerve, but no neurones were found either in the band or nearby, so the source of the fibres in the telotroch nerve remains unknown. In addition to having different innervation, the two bands also respond differently to cholinergic agonists, which elicit telotroch arrests but have no visible effect on the circumoral band. The oesophagus has a well‐developed musculature and an extensive nerve plexus. During feeding, the oesophagus repeatedly contracts, forcing excess water out along two lateral channels prior to swallowing. These channels are also sites of gill slit formation, so there is evidently a continuity between the water bypass mechanism of the larva and that of the postmetamorphic juvenile.

The food-collecting and waste-rejecting systems of the tornaria larval stages of enteropneust hemichordates are similar to those of larval and adult lophophorates and adult pterobranch hemichordates. Water entering the oral grooves is deflected towards the mouth and the impetus of heavy, potentially inedible particles may take them across the flow lines of the water currents inferred from the movements of suspended particles to impinge on cilia which reject them into the outgoing water currents. Lighter, potentially edible material remaining suspended in the deflected water currents is intercepted by cilia on an oral hood which is similar in structure and function to the preoral lobe of the actinotroch larvae of phoronids. Excess water carried into the mouth by cilia on the dorsal surface of the esophagus is rejected via lateral grooves which develop into pouches prior to metamorphosis. Following metamorphosis the pouches make contact with the body wall to form gill slits which continue to allow water to escape from the pharynx. This finding that the function of allowing excess water to escape is performed by lateral grooves in the esophagus of tornariae supports previous speculations on the evolution of gill slits and provides further evidence for relationships between lophophorates, hemichordates, and chordates.

Potentially inedible particles entering the mantle cavity of Glottidia pyramidata impinge on the frontal surfaces of ungrooved outer tentacles and are removed very efficiently by frontal cilia beating directly towards the exhalent aperture. Particles which settle out of the inhalent current are resuspended by cilia on the mantle ampullae. Edible material passing between the outer tentacles is captured by the combined action of laterofrontal cilia beating into the grooves and frontal cilia beating towards the bases of the inner grooved tentacles of the spirolophous lophophore. The arrangement of the tentacles and their cilia on the spirolophe of G. pyramidata is contrasted with the plectolophous condition in the articulate brachiopod Laqueus californianus in which heavy potentially inedible particles penetrate deeply into the lophophore before being trapped by ungrooved tentacles which, because of differences between the developmental patterns of articulates and inarticulates, lie within the grooved tentacles. Furthermore, the immobile laterofrontal cilia on both sets of tentacles of L. californianus detect waste particles. It is suggested that the exterior position of the waste-rejecting tentacles and the involvement of the laterofrontal cilia in food collection in lingulacean inarticulates may have contributed to their ability to compete successfully with bivalve molluscs in turbid habitats and their persistence in the fossil record.