ИСТИНА

Innervation of unpaired branchial appendages was studied using confocal laser scanning microscopy in two annelid species: Terebellides cf. stroemi (Trichobranchidae) and Cossura pygodactylata (Cossuridae). It was shown that the branchial filament is innervated by one unpaired segmental nerve coming from the ventral nerve cord on the level of the chaetiger 2 in C. pygodactylata. This nerve goes transversally along the body wall and come to the base of the branchial filament from the side. This fact allowed us to conclude that 1) the branchial filament is unpaired structure, not representing merged paired branchiae and not connected with prostomial and peristomial appendages in origin; and 2) the branchial filament is a result of a shift of one of paired segmental branchiae with parallel reduction of the second branchia from this pair. The branchial organ of T. cf. stroemi is innervated by paired segmental nerves from the first and the second chaetigers. The segmental nerves are connected by longitudinal nerves going on lateral sides of the body closer to the dorsum. One nerve from each side proceeds to anterior part of the branchial stalk and one nerve to posterior. All these nerves are linked by transversal connections. Anterior stalk nerves come into anterior branchial lobes and posterior nerves to posterior lobes. In the branchial lobes, main nerves give rise to thinner ones, going to each of branchial lamellae. The present study confirmed a paired nature of the branchial organ of Terebellides and its origin from two pairs of segmental branchiae