Locomotory and feeding effectors of the tornaria larva of Balanoglossus biminiensis

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.