132 THE EYE IN EVOLUTION 



of the siphon ; again, each contains an optic organelle with a surrounding 

 nerve-plexus (Light, 1930). It is interesting that somewhat similar cells, 

 presumably sensitive to light, have been described in the epidermis of the tail 

 of the ammocoete larva of the lamprey (Steven, 1951).^ 



Apathy. Mitt. zool. Stat. Neapel, 12, 495 Kowalski. La Cellule, 25, 291 (1909). 



(1897). Light. J. Morph. Physiol., 49, 1 (1930). 



Beer. Wien. klin. Wschr., \^, 255, 29,5, Minchin. Proc. roj/. 5'oc. J5, 60, 42 (1896). 



314(1901). Steven. Quart. J. micr. Sci., 92, 233 



Hess, W. N. Biol. Bull., 38, 291 (1919). (1951). 



J. Mor/j/i., 39, 515(1924) ; 41,63(1925). Whitman. Zool. Jb., Abt. Anat., 6, 616 



Hesse, R. Z. m«s. Zoo/., 58, 394 (1894) ; (1893). 



81, 393 (1896) ; 62, 671 (1897) ; 63, Worley. Proc. nat. Acad. Sci., 19, 323 



361(1898); 65,446(1899); 72,565 (1933). 



(1902). J. exp. Zool., 69, 105 (1934). 



Die Sehen der niederen Tiere, Jena J . cell. comp. Physiol., b,5Z {l^^'i) ; 18, 



(1908). 187 (1941). 



THE MULTICELLULAR SIMPLE EYE 



While the most primitive example of the simple eye is represented 

 by a single light-sensitive cell, the next obvious development is the 

 association of a group of epithelial cells each reinforcing the effective- 

 ness of the others. For this purpose several evolutionary lines have 

 been followed so that eventually the end-organ appears to reach a 

 degree of complexity greater than the analysing capacity of the nervous 

 organization. Efficiency is enhanced not only by the progressive 

 development of the capacity to form detailed images as the number of 

 sensory cells increases, but also of the ability thereby obtained to 

 localize the stimulus in space and analyse the visual field (a directional 

 eye). The association of pigment forming an absorbent screen within 

 or around the sensory cells is a constant feature, while the efficiency of 

 the organ is further increased by the development of a focusing 

 apparatus. To this end a wide variety of optical mechanisms is 

 exploited varying from a pin-hole to a lens-system of progressive 

 elaboration until, in Cephalopods. a dioptric mechanism comparable to 

 that of Vertebrates is reached. The inner ends of the sensory cells are 

 prolonged to form elongated processes or nerve fibres which leave the 

 deep surface of the ocellus to join a subepithelial plexus or a ganglion. 



The sensory cells usually remain in association with the surface 

 layer but occasionally migrate inwards to the subepithelial tissues ; 

 and since the latter type of ocellus undergoes less evolutionary develop- 

 ment than the former, we will discuss it first. 



The Subepithelial Eye 

 The fnigration of a number of light-sensitive cells from the surface 

 with th( ■ aggregation in the subepithelial tissues to form a sub- 

 epithelial > always results in an organ of a very elementary type. 



1 p. 263. 



