ИСТИНА

Caobangia – an aberrant group of shell-burrowing annelids living on tropical freshwater snails. In 1974 M.L. Jones described caobangiid larva and the curious phoronid-like anatomy of adults; but he did not observed the development of organs during the metamorphosis. Laboratory observation on the development of Caobangia billeti allow us to detail undescribed metamorphosis stages and to throw some light on the organs anlage, ascending gut formation and localization of the prepygidial growth zone. C. billeti trochophore consists of 10 segments (prostomial, peristomial and 8 larval body segments) and the pygidium. Embryogenesis involves a development of segmentally arranged bands of cilia and ventral ciliar fields, development of chaetae and the beginning of anterior radioles formation. During the metamorphosis the branchial crown develops, pygidium accretes with the dorsal side of segments 7–11. It leads to formation of the anal trunk with the dorsal anus near to the 7th segment. Prostomial and peristomial segments fuse together and reduce; cephalic ganglia descend into the first thoracic chaetiger. The post–metamorphosis juvenile has 8 thoracic segments with 10 pair of ganglia and 3 abdominal segments, lacking the ventral nerve cord. The only segmentation evidence in the abdomen are 3 pairs of segmental parapodia. As the worm grows, the anal trunk stretches along the body and the number of abdominal parapodia increases posteriorly. Thus, the prepygidial growth zone lies at the posterior end of the abdomen, and the ascending gut is a projection of pygidium.