The digestive tract of actinotroch larvae (Lophotrochozoa, Phoronida): anatomy, ultrastructure, innervations, and some observations of metamorphosis

The digestive tract of actinotroch consists of the vestibulum, oesophagus, stomach with stomach diverticulum, midgut, and proctodaeum. Monociliate muscle cells resting on the basal lamina of the oesophagus form its circular musculature. The epithelium of the cardiac sphincter contains axonal tracts and neurosecretory cells. Glandular, secretory, and digestive cells form the epithelium of the stomach and stomach diverticulum. The epithelium of the midgut is biciliate. The proctodaeum is divided into two parts, differing in fine structure and function. Individual serotonian and FMRFamide neurons and fibers occur in the oesophagus, cardiac sphincter, and midgut, as well as surrounding the anus. In larvae of Phoronopsis harmeri Pixell, 1912 during metamorphosis, the larval oesophagus gives rise to the juvenile oesohagus, the upper portion of the stomach stretches and transforms into prestomach, the stomach diverticulum moves into the stomach and then is digested, the larval stomach becomes the juvenile stomach, the midgut gives rise to the pyloric region, and the proctodaeum transforms into the ascending branch of the juvenile digestive tract. The data do not support the views that the proximal part of adult digestive tract forms from the ectodermal epithelium of the dorsal and ventral epidermis of the larva or that the telotroch enters the intestine during metamorphosis.