128 



THE EYE IN EVOLUTION 



Figs. 82 to 85. — Types of Bipolar Visual Cell. 



Fig. 82. 



Fig. 83. 



J. 



Fig. 84. 



Fig. 85. 



Fig. 82. — The visual cell of the edible snail, Helix pomatia, showing 

 cilia (after Hesse). 



Fig. 83. — The visual cell of the grey slug, Llnuix maximus, showing 

 elongated end with cilia (after Hesse). 



Fig. 84. — The visual cell of the marine worm, Phniaria forva, showing 

 brush-like border (after Hesse). 



Fig. 85. — Rod-like visual cell of the Tabanid fly, Cliry.b-ops marmoratus 

 (after Ciaccio). 



specialized derivative 

 in many ectodermal 

 several types of sensor 



Fig. 86. — Apolar Visual 

 Cell. 



A light-sensitive cell in 

 the earthworm, LiDtibvi- 

 cus terre.stris, stained with 

 silver nitrate. N, nu- 

 cleus ; OX, optic nerve 

 which breaks up into a 

 network of neurofibrils, 

 CN, throu;^: out the cyto- 

 plasm ; ()(i, optic orga- 

 nelle coverev a, denser 

 network of urofibrils 

 to form ti ■.'tinella 

 (after W. N. li ). 



of the first. 1 cilia (Figs. 82 and 83) are found 

 cells and form an important differentiation of 

 y cells, and it may be, as Hesse (1902) surmised, 

 that they represent the distal terminations of 

 bmidles of the " neuro-fibrillse " which form the 

 primitive conducting mechanism ^ ; a similar 

 configuration is seen in cells with a striated or 

 brush-like border (Fig. 84). rods (or rhabdites) 

 appeared originally as simple but stouter 

 cylindrical prolongations of protoplasm, clear 

 and refractile in nature, which in subsequent 

 evolution have undergone innumerable speciali- 

 zations (Fig. 85) ; they are found in worms, 

 Arthropods, and Molluscs, and they attain 

 their highest differentiation in the rods and 

 cones which form the imique receptor apparatus 

 in the eyes of Vertebrates. 



The second type of recejDtor is seen among 

 worms and Molluscs ; in it the cell is apolar in 

 its general arrangement but contains a light- 

 sensitive mechanism within the cell body (Fig. 

 80). This typically takes the form of a 



1 p. 243. 



2 See VVorley (1933-41). 



