Nuchal Organs of the Polychaete Parapionosyllis manca (Syllidae)

Abstract

Nuchal are known to be important sensory of many polychaete annelid species. This study describes the fine structure of the nuchal and associated glands of Parapionosyllis manca. In each nuchal organ, the epithelium of the nuchal cleft encloses an olfactory chamber through which sensory cilia and microvillus-like processes radiate. These sensory components originate from bulbs of sensory cells that are closely associated to form a sensory bundle. Each bundle of cells is encased in a palisade of support cells bearing motile cilia. Numerous secretory vesicles occur in the cytoplasm of the support cells and beneath the modified cuticle of the nuchal organ. The support cells of the olfactory region extend posteriorly to form the nuchal lobes, or glands. The cells are elongate and taper to a thin neck just before leaving the sensory area of the nuchal organ. There are essentially no nuclei in the sensory region as these and most of the other cellular organelles are concentrated in the lobes that extend into the body cavity but are contained by the epidermal basement matrix. The nuchal organ is innervated by the nuchal nerve, which extends posteriorly from the hind portion of the brain. Additional key words: ultrastructure, sensory organs, Annelida The nuchal of polychaete annelids are a pair of pits located in the head region of the animal, usually between the prostomium and the peristomium. They are believed by most investigators to be chemosensory (Lang 1888; Rolleston & Jackson 1888; Rullier 1950, 1951; Bullock & Horridge 1965; Purschke 1986, 1990; Schlotzer-Schrehardt 1986; Rhode 1989, 1990) and probably function in the location of food. Others believe the may also have a role in coordinating reproductive behavior (Schlotzer-Schrehardt 1987). The present study describes the nuchal of Parapionosyllis manca, an interstitial polychaete belonging to the family Syllidae. The nuchal organ, found in many families of polychaetes with varying lifestyles, was first described in 1849 by J.H. Huxley while on an expedition aboard the H.M.S. Rattlesnake. He collected two specimens of Tomopteris onisciformes (Tomopteridae) and noted the presence of a pair of rounded elevations running back along the dorsal surface in the form of a band or ridge between the narrow neck and the second pair of cephalic appendages. This information was published by Carpenter (1859). A year later, the were more a To whom correspondence should be addressed. Present address: Dept. of Companion Animal and Special Species Medicine, North Carolina State University, College of Veterinary Medicine, Raleigh, NC 27606, USA. accurately described by Carpenter & Claparede (1860), and were called ciliated epaulettes. Several decades later workers began to speculate on the function of nuchal organs. Lang (1888) referred to nuchal as of smell. Rolleston & Jackson (1888) stated that among their organs of special sense, many polychaetes had grooves supplied by nerves from the cerebral ganglia. Malaquin (1893) considered the nuchal of syllids to be sense responsive