Dorsolateral ciliary folds in the polychaete foregut: Structure, prevalence and phylogenetic significance

Ciliary folds form the dorsolateral walls of the foregut in numerous polychaetes. These feeding structures have not been recognized earlier. They are described here for 26 species in 16 families. The folds consist of ciliated cells, usually associated with gland cells, and have no intrinsic muscular system. Protraction of the dorsolateral folds to make contact with the substratum during uptake of food is mainly achieved by contractions of the musculature of the body wall in the anterior part of the body. These folds either occur alone or are associated with a ventral pharyngeal organ. Dorsolateral ciliated folds are structures originally adapted to microphagy. From the present study and the literature it is obvious that these structures are widespread among polychaetes of various taxa. This distribution and their similar structure suggest that dorsolateral folds are phylogenetically old structures which might already have been present in the stem species of polychaetes.