WORMS 



191 



Thvis W. N. Hess (1925) found that in the prostomium there were some 

 440 light-sensitive cells in the epidermis and 700 sitviated in nearby nerve 

 enlargements, while in subsequent segments they were much fewer. Their rela- 

 tive numbers in corresponding sinall areas (200 x SOOti.) on the dorsal surface of 

 the animal are as follows — in the prostomium, 18 ; 1st seginent, 10 ; 2nd segment, 

 5 ; 3rd segment, 3 ; 40th segment, ; antepenultimate segment, 1 ; penultimate 

 segment, 1 ; last segment, 4. The segiuental photic sensitivity varies directly 

 with the number of receptors, and the distribution of light-sensitive elements 

 conforms with the habits of the earthworm. ^ 



Among POLYCH.ETE woEMS. the burrowing lob-worm, Are^iicola 

 marina, is not provided with visual organs although the prostomial 



Fig. 173. — The Head of Nereis, Showing the Four Eyes. 



e, eyes ; j, jaw ; p, palp ; pe, peristomium (first two segments fused) ; 

 ph, pharynx ; pp, first ordinaiy paraijodium ;\ pr, prostomium; t, accessory 

 teeth ; tc, tentacular cirri ; te, tentacle. (From Borradaile's Manual of 

 Elementary Zoology ; Oxford University Press.) 



lobes are diffusely sensory. In contrast with the burrowing type, 

 however, the freely-swimming marine polychaetes show a much richer 

 development (Fig. 173). Of these, Nereis is a typical example. This 

 worm has four prominent eyes situated on the prostomium, each of 

 the cupulate type with a cuticle externally and a retina internally 

 formed of well-developed sensory cells with rod-like receptor endings 

 (Fig. 101). Other forms, such as Polyoplithalmus, have in addition to 

 the prostomial eyes similar pairs of subepithelial organs in many 

 segments of the body ; such eyes ^ are formed sometimes on each 

 segment {Myxicola (esfhetica ; Eunice), and occasionally on the anal 

 segment {Fabricia). 



A much more complex type of e}e of the vesicular type is found 



1 p. 572. 



^ These organs, usually considered to be " eyes " are said by some to be liglit- 

 producing (p. 736) (Benham, 1896). 



Nereis 



