THE EYES OF ANNULATA 97 



The eyes of these different classes of " ringed worms " are mostly 

 of the simple upright type ; and when present in the adult animal are 

 mostly borne on the anterior segment of the head (prostomium) ; they 

 are usually paired and near the median plane. The eyes in some species 

 are exceptional in position and also in structure. Thus among the 

 Polychaete worms, Polyophthalmos, in addition to the prostomial eyes, 

 has pairs of luminous organs on many segments of the body ; while 

 Leptochone has a pair on each segment and in Fabricia there is a pair on 

 the anal segment. Moreover, in many species of Sabella and all the 



re. rd 



Fig. 63. — Section through One of the Eyes of Nereis, showing a Vitreous, 

 Non-cellular Type of Lens, which is continuous with the Vitreous 

 Body contained in the Optic Cup. (After Andrews.) 

 co.: cornea. rd. : layer of rods. 



cut. : cuticle. p. : pigment. 



/. : lens. re. : sensory cells of retina. 



species of Dasychone, eyes are present on the branchial filaments. These 

 are highly differentiated compound eyes. The eyes of leeches are peculiar 

 in structure, being formed of a number of large refractile cells lining a 

 tubular pit and surrounded by a pigment layer. Moreover, the nerve fibres 

 and cells, instead of being situated on the outer side of the pigment layer, 

 are on the inner side of the refractile cells and lie in the central axis of the 

 tube in what appears to be its potential cavity (Fig. 66). Eyes are absent 

 in many of the adult animals, e.g. the earth-worms and certain of 

 the parasitic and fixed forms, such as Terebella and Serpula. The 

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