230 



THE EYE IN EVOLUTION 



by the experiments of Nagel (1896) and Hesse (1898) 

 and more particularly by those of Parker (1908) and 

 Crozier (1917). There would seem little doubt that 

 it is not a vestigial eye but that its function is to 

 endow the infundibular organ with directional ability 

 by casting a shadow upon it when the animal or the 

 light source moves, a primitive role we have already 

 seen in the eye -spot of the Protozoon, Euglena ^ 

 (Franz, 1912-34 ; WoUenhaupt, 1934). 



A second jDhotosensitive mechanism is seen in the 

 ORGANS OF HESSE (1898), individual cells scattered on 

 the ventral and lateral aspects of the nerve-cord to- 

 wards its posterior end (Figs. 238 and 239). These 

 are single large ganglion cells variously orientated, 

 each provided with a brush-like ciliated margin and an issuing nerve-fibre, each 

 capped by a crescent-shaped pigment cell to give it directional a.bility. The 

 distribution and structure of these unique cells have been fully studied by a 

 number of observers (Franz, 1923 ; Joseph, 1928 ; Kolmer, 1928 ; WoUenhaupt, 

 1934) and their photosensory function established by Parker (1908) and Crozier 

 (1917). 



Fig. 238. — Visual Cell 

 of auphioxvs. 



n, nucleus ; c, striated 

 margin ; p, jjigment 

 mantle. 



Fig. 239. — The Neural Visual Cells of Amphioxus. 



Section through the sj^inal cord in the region of the 5th segment, showing 

 the central canal, C, and the large visual cells of Hesse, H, with their associated 

 pigment cells (after Hesse). 



Ayers. ZooZ. ^Inz., 13, 504 (1890). 

 Boeke. Anat. Anz., 32, 473 (1908). 

 Crozier. Anat. Rec, 11, 520 (1917). 

 Edingef. Anat. Anz., 28, 417 (1906). 

 Franz. /?-o/. Zbl., 32, 375 (1912). 

 Jena. Z. Natzirw., 59, 401 (1923). 



Bolks . 

 Berl,. 



vergl. Anat. d. Wirb'lliere, 

 (ii), 989 (1934). 



Froriep. Handbuch d. vergl. und exper. 

 Entwicklungslehre d. WirbeUiere, Jena, 

 2 (1906). 

 Anat. Anz. (suppl.), 29, 145 (1906). 

 Hesse. R. Z. wiss. ZooL, 63, 456 (1898). 

 Joseph. Anat. Anz. (suppl.), 25, 16 (1904). 



Biol, generalis, 4, 237 (1928). 

 Kohl. Zool. Anz., 13, 182 (1890). 



126. 



