The nuchal organs of polychaetes from four different families (Nereidae, Nephtyidae, Phyllodocidae and Glyceridae) were examined with the light and electron microscopes. In each case, the organ consists of ciliated cells and primary sensory elements. The ciliated cells are similar to the cells of the adjacent epidermis but bear motile cilia. Primary sensory neurons are situated within the organs in Nephtyidae and Phyllodocidae, but are located within the brain in Nereidae and Glyceridae. Each sensory cell gives rise to a distal process which penetrates between the ciliated cells to form an apical sensory bulb bearing modified cilia. Apically these processes are lined with juxtamembranous plaques. The ciliated cells are innervated by efferent axons from the brain, and in Nereis the axons appear to be peptidergic. The elements comprising the nuchal organs closely resemble those of the vertebrate olfactory mucosa.Nuchal organs are epidermal sensory structures located within the prostomium or peristomium of most polychaetes. Their exact position and degree of morphological development show considerable variation between individual species (reviews by Fauvel, '59; Bullock and Horridge, '65). They may be simple pits at the posterior edge of the prostomium (as in the Terebellidae, Sabellidae and Sabellariidae), whereas in Spionidae, Amphinomidae and others, they form complex folds that extend over several segments. The greatest degree of morphological development is exhibited by members of the Opheliidae, Capitellidae and Ariciidae. The sensory epithelium can be fully exposed to the environment in a number of ways, e.g., flexure of the body wall or eversion by hydrostatic pressure (Fauvel, '59). Eversion of the organs in Nephtys has been reported by Clark ('58).The numerous histological investigations, supported by a small number of experimental studies (Rullier, '50a,b, '54), have been the basis of the assumption that nuchal organs are chemoreceptors which aid in the detection of food. The present ultrastructural study concerns representatives of four different polychaete families, whose nuchal organs exhibit structural variations and which, in particular, differ with respect to the location of their primary sensory cells.J. MORPH., 144: 167-184.
MATERIALS AND METHODSSpecimens of Nephtys caecu were collected from Tynemouth Bay, Northumberland, and excised prostomia fixed in either (a) 4% glutaraldehyde in sea water, followed by 2% osmium tetroxide in sea water (pH adjusted to approximately 7.59, or (b) 2 % osmium tetroxide in 0.1 M phosphate buffer (pH 7.2-7.5) containing 25% sucrose. Specimens of Eulalia viridis were collected from Cullercoats Bay, Northumberland, and excised prostomia fixed in 2% osmium tetroxide in 0.2 M sym-collidine buffer (pH 7.4) containing 25% sucrose. Specimens of Nereis diversicolor were collected from the River Avon, Bristol, and prostomia were fixed in 1% osmium tetroxide in veronal acetate buffer (pH 7.4). Specimens of Glycera rouxi were collected subtidally off the Northumbrian co...
