Ciliary bands in the prostomium region of Dimorphilus gyrociliatus (Annelida: Dinophiliformia) and their involvement in food uptake

: A prototroch is a characteristic larval idiosyncratic structure conserved among Lophotrochozoa. It appears as the first prominent structure during embryogenesis, serves for swimming and filter-feeding, then degenerates or is modified after metamorpho-sis. Using cutting-edge confocal laser scanning microscopy, we investigated the detailed structure of the Dimorphilus gyrociliatus prototroch area and demonstrated that it is much more complex than previously thought. The first row of ciliated cells differentiates at the trochophore stage. By juvenile stage, three rows of ciliated cells are located in the prostomium region: an anterior row of cells with long (14–16 µm) cilia, an intermediate row of cells with short (3–4 µm) cilia, and a posterior row of cells with cilia of middle length (9– 10 µm). Each ciliated row consists of two bands of cilia-bearing cells. We suggest that the anterior row represents what is commonly considered a prototroch. At the same time, the intermediate row is considered homologous to the adoral ciliary zone, and the posterior row is considered homologous to the metatroch. Older individuals retain this ciliated apparatus until the last days of their life. With age, cilia in prototroch and metatroch become twice shorter; and the prototroch gets wider, at the end consisting of four lines of ciliated cells. inhibition of muscle contraction, the freshly hatched juveniles maintain the ability to uptake food particles. These findings confirm earlier observations showing that D. gyrociliatus uses its complex ciliated apparatus in the prostomial region for filter-feeding in addition to bulbus-scrabbing. Thus, we clarified the organization of the ciliary bands of D. gyrociliatus located in the prostomial region, documented their persistence during ontogenesis, as well as confirmed the participation of the ciliary apparatus in active food intake. These data support the paedomorphic character demonstrated for Dimorphilus gyrociliatus .