Pharynx and intestine

Abstract

The alimentary canal of polychaetes consists of a foregut, midgut, and hindgut. The alimentary canal shows different specializations even in homonomously segmented polychaetes. The foregut gives rise to the buccal cavity, pharnyx and oesophagus, the midgut may be divided into a stomach and the intestine proper. Since polychaetes use a wide spectrum of food sources, structures involved in feeding vary as well and show numerous specializations. In the foregut these specializations may be classified as one of the following types: dorsolateral folds, ventral pharynx, axial muscular pharynx, axial non-muscular proboscis and dorsal pharynx. The latter, typical of oligochaetous Clitellata, occurs rarely in polychaetes. The structure, evolution and phylogenetic importance of these different types are described and discussed. Axial muscular and ventral pharynges may be armed with jaws, sclerotized parts of the pharyngeal cuticle. Terminology, structure, occurrence and development of the jaws are briefly reviewed. Special attention has been paid to the jaws of Eunicida including extinct and extant forms. Conflicting theories about the evolution of the jaws in Eunicida are discussed. The epithelia of the intestine may form a pseudostratified epithelium composed of glandular cells, absorptive cells and ciliated cells or only one cell type having similar functions. A conspicuous feature in the intestine of certain polychaetes is the occurrence of unicellular tubular structures, called enteronephridia. So far these enteronephridia are only known in a few meiofauna species.