Ultrastructural observations on the gills of polychaetes

Abstract

The gills of several polychaete species belonging to 9 families were studied by scanning and transmission electron microscopy. The surface epithelium is covered by a thin cuticle which is invaded by microvilli penetrating the epicuticle in certain species. Some epithelial cells bear cilia, others are mucus-producing cells. The ciliary cells may be arranged in rows and maintain a constant flow of water over the gills. The distance between external water and blood stream differs considerably according to the species investigated. InMalacoceros the gills are characterized by closed afferent and efferent subepithelial vessels, which correspond to tubular invaginations of the coelomic wall. These vessels are lined by the basement lamina of the coelothelial cells, which are of the epitheliomuscular type. The vessels are open in the gills of other polychaetes and release the blood stream into a system of spaces immediately below the epidermis (e.g. in the branchial lamellae ofPectinaria andTerebellides). In several species the blood comes into very intimate contact with the cuticle (e.g. in the gill filaments ofDendronereides), but also in these animals both are separated by a very small epidermal layer.