412 Worms, Annelida p s p lant 1st mant tc pr 1st gi pe 1st dc h t el (b) (a) ci p t (c) pr p pe 1st p gi pr 1st 1st pr/pe t (d) (f) gi (e) Figure Polychaete anterior ends (a) Polynoid, dorsal view (b) Sabellariid, lateral view (c) Sabellid, lateral view, whole body (d) Siboglinid, dorsal and ventral view (e) Spionid, dorsal view (f) Terebellid, lateral view Modified from Rouse GW and Fauchald K (1997) Cladistics and polychaetes Zoologica Scripta 26(2): 139–204 Sense Organs Eyes are mostly eye-spots located on the prostomium or even directed on the upper surface of the brain In some polychaetes the eyes become much larger when the species are sexually mature (e.g., nereidids), and the epidermis over the eye becomes a translucent lens In a group of pelagic polychaetes (e.g., alciopin phyllodocids) the eyes are a very large camera construction with a distinct lens and with focal capabilities of considerable complexity In the polychaete eyes, the sensory cells penetrate through a pigment layer (retina) into the space in which the lens is supported and the optical nerve is collected behind the pigment layer, thus the retina is reversed compared to vertebrate eyes The nuchal organs are usually located in pits, in short grooves or on short ridges near the junction between the prostomium and the first segment The nuchal organs may be very large, complexly folded structures (caruncles) in the euphrosinids and amphinomids And the epaulets present in some syllids are also nuchal organs In some spionids the nuchal organs stretch along most of the anterior region of the worm as narrow, ciliated attached ridges The presence of nuchal organs has been considered diagnostic for the polychaetes in relation to the clitellates, which lack these