Sense organs in polychaetes (Annelida)

Polychaetes possess a wide range of sensory structures. These form sense organs of several kinds, including the appendages of the head region (palps, antennae, tentacular cirri), the appendages of the trunk region and pygidium (parapodial and pygidial cirri), the nuchal organs, the dorsal organs, the lateral organs, the eyes, the photoreceptor-like sense organs, the statocysts, various kinds of pharyngeal papillae as well as structurally peculiar sensory organs of still unknown function and the apical organs of trochophore larvae. Moreover, isolated or clustered sensory cells not obviously associated with other cell types are distributed all over the body. Whereas nuchal organs are typical for polychaetes and are lacking only in a few species, all other kinds of sensory organs are restricted to certain groups of taxa or species. Some have only been described in single species till now. Sensory cells are generally bipolar sensory cells and their cell bodies are either located peripherally within the epidermis or within the central nervous system. These sensory cells are usually ciliated and different types can be disinguished. Structure, function and phylogenetic importance of the sensory structures observed in polychaetes so far are reviewed. For evaluation of the relationships of the higher taxa in Annelida palps, nuchal organs and pigmented ocelli appear to be of special importance.