ИСТИНА

Cossurids have rather unusual type of feeding apparatus with oral tentacles attached to the dorsal wall of the buccal cavity. More than 100 juvenile and adult specimens of C.pygadactylata on different stages of protrusion of tentacles were studied using phalloidin-labelling and cLSM and semi-thin sectioning. Early juveniles do not possess tentacles and circumbuccal complex (and mouth opening?). In adults 4-10 tentacles arise as paired structures. Each tentacle contains one thick and two thin longitudinal muscle filaments; thick muscle fibers from all tentacles merge forming left and right tentacle protractors rooted in dorsal longitudinal bands of the body wall. Circumbuccal complex includes well developed upper and lower lips. They contain outer layer of transversal fibers and lower lip also contains inner diagonal muscles going to ventral longitudinal bands. Upper lip attached to dorsal longitudinal bands by median situated dorso-ventral muscle and to prostomium musculature by thinner unpaired fiber. Lower lip attached to the dorsal body wall by paired transversal muscles. It can be supposed that opening of the mouth and protrusion of tentacles occurs when dorso-ventral muscle of upper lip and anterior unpaired fiber contract and pull upper lip forward and up and diagonal muscles contract and pull lower lip backward. Possibly transversal muscles of the body wall going between dorsal and ventral longitudinal bands also take part in protrusion of oral tentacles. Inverse process of retracting of tentacles occurs due to contraction of tentacle retractors and transversal fibers of circumbuccal complex. In specimens with fully protruded tentacles it is evident that they have non-muscular axial proboscis with dorso-ventral asymmetry: its dorsal part is enlarged and transformed to tentacles. Similar type of proboscis is observed in some Opheliidae. The most probably these structures evolved independently in both families; further ultrastructural investigations are required.