Evidence for a transient digestive tract in Vestimentifera

A medial process has been observed at the base of the branchial plume of juvenile vestimentiferans; this structure persists for only a short period in early developmental stages of the worms. In the youngest specimens observed there is a continuity from the distal, ciliated opening of the process to a posterior basal opening. This complete apparatus appears to be the mouth-gut-anus of the Vestimentifera, heretofore not observed. The ciliated opening (mouth) of the process connects with a ciliated duct (esophagus/foregut) which, in turn, leads to the presumptive trophosome (midgut), then into a spacious, thin-walled region (hindgut) and, finally, opens to the exterior via a posterior, terminal anus. In later stages the midgut is filled with presumptive endosymbiotic bacteria; its communication with the hindgut is lost, and the anal opening disappears.