ИСТИНА

Chaetae in polychaete parapodia are usually arranged in straight, curved, crescent or spiral rows. Throughout the worm’s life chaetae may renew for a number of reasons: larval metamorphosis, as a result of regeneration, epitoky or simply with age. As a rule, formation of new chaetae is limited to one edge of the row in the formative site. Newly forming chaetae enter the row and push the older ones out; therefore, individual chaetae move gradually from the formative site to the opposite edge of the row where the degenerative site may occur. We have examined ultrastructure of chaetal sac and chaetal replacement and displacement of capillary chaetae, hooked chaetae and uncini in the polychaete families Oweniidae, Maldanidae, Terebellidae, Sabellidae and Serpulidae. The chaetal sac is cowered by the basal membrane and may contain one or more chaetal rows, each row having its own formative site. The formative site is present in each chaetal row, whereas the degenerative site is difficult for identification and may be absent in some species. Generally, distinguishable degenerative sites were described for hooked chaetae in chaetal rows in Maldanidae and Capitellidae and in chaetal patches in Oweniidae. Perhaps, the efficiency of formative and degenerative sites depends on the chaetal sac structure.