THE SIMPLE EYE 



131 



is similarly a single photosensitive neural cell associated with a pigment mantle 

 (Fig. 238). 



SINGLE APOLAR VISUAL CELLS are typified in the light-sensitive 

 organs of the earthworm, Limibricus terrestris ; these have received 

 closer study than those of any other species (R. Hesse, 1894—97 ; Beer, 

 1901 ; Kowalski, 1909 ; W. N. Hess, 1924-25) (Figs. 86 and 88). 

 They are found in two sites — ^in the epithelium and in association with 

 the nerves immediately underneath ; 

 it is probable, as has been shown in 

 the medicinal leech (Whitman, 1893), 

 that the latter originated in and 

 migrated from the epidermal layer. 

 In appearance they are distinctive. 

 The superficial cells are small and 

 rounded, lying at the base of the 

 epithelium and into each the sub- 

 epithelial nerve-net sends a nerve- 

 fibre which breaks up into a network 

 of neurofibrils surrounding the ellip- 

 soidal optic organelle ^ ; the sub- 

 epithelial cells clumped around 

 enlargements of the nerve plexus are 

 similar in type and presumably in 

 function. It is interesting that a 

 dense layer of pigment lies under the 

 epithelium apparently unassociated 

 with the light-sensitive cells ; but as 

 they traverse this layer and run into 

 the epithelium, the nerves make pin- 

 point openings in the dense pigmen- 

 tary blanket so disposed that incident 

 light will enter, dorso -anteriorly at 

 the anterior end of the worm and 

 dorso -posteriorly at the posterior end, 

 and will thus strike the subepithelial cells directly as either extremity 

 emerges from the burrow. 



The light-sensitive cells of leeches are also of a very similar type, each 

 containing an identical optic organelle supplied with a nerve fibre from the 

 dorsal ganglion (R. Hesse, 1897). They may occur as isolated cells just below 

 the epithelium or may lie in association with other sensory cells. ^ Light- 

 sensitive cells identical with those of the earthworm are also found in lamelli- 

 branch molluscs ; thus in the clam, Mya arenaria, they are seen, plentifully 

 supplied with nerves, lying jvist beneath the epithelial layer on the inner surface 



1 p. 128. 2 p. 1.3.3. 



Fig. 88. — Single Light-sensitive 

 Cells in the Earthworm, Lim- 



BRICVS TERllKSTRIS. 



The photoreceptor cells, L, lying in 

 tlie basal region of the epidermis, E, 

 and also in enlargements of the nerve 

 in close relation to the eijidermis. The 

 nerve is seen to spread out beneath 

 the epithelium as a subepidermal 

 nerve plexus, the fibres of which go to 

 the photoreceptor cells. C, cuticle 

 (after W. N. Hess). 



4fipii^i4uiuiS$&^^l^ 



Lumbricus 



Mya arenaria 



