ИСТИНА

Cossuridae comprises only one genus with 26 species; they are small and medium size worms inhabiting soft sediments from intertidal to abyssal depths; cossurids easily recognizable by their unpaired branchial filament. Despite of their commonness, Cossuridae are poorly known group with many unresolved problems. Relationships within family as well as with other polychaetes are uncertain; molecular data are scarce. Some species such as C.pygodactylata have a wide geographic and ecological range, it suggests the presence of potential cryptic species. Identification of cossurids may be complicated: they have comparatively poor external morphology - rather uniform simple chaetae, lack of head appendices, similar body regionalization. The important characters used for distinguishing of species such as attachment of branchial filament and number of thoracic chaetigers are hard to interpret correctly. Homology of branchial filament with palps, dorsal cirri or branchiae of other polychaetes is unknown. To reveal the nature of branchial filament we analysed its innervation using immunohistochemistry and confocal laser scanning microscopy. It was shown that branchial filament is innervated from left segmental nerve going from the ganglion of chaetiger 2. This result proves the absence of homology of filament with palps. The most possible homology is with segmental structures – dorsal cirri or branchiae, like in cirratulids. The current state of our knowledge does not allow hypothesizing on phylogenetic affinity of Cossuridae; further morphological and molecular studies are needed.