INVERTEBRATE EYES 127 



stigma may be more complex. In Pouchetia, for example, the pignient is arranged 

 in the form of a cup-shaped mass the opening of which is covered by a refractile 

 structure, while within the cup, between the primitive lens thus formed and the 

 pigment, lies the light-sensitive protoplasm (Fig. 81) (Schiitt, 1896) ; occasionally 

 in the marine forms this structure is of such a size that it has been called an 

 ocellus, but it is acellular. In all these cases the organelle combines photo- 

 sensitivity with directional detection in order to perform its phototactic function. 



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2Q 387 (188'^r Mast and Johnson. Z. vergl. Physiol., IQ, 



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N.Y. (1911). 463(1900). 



LIGHT-SENSITIVE CELLS 



Once multicellular organisms evolved, the obvious specialization 

 occurred whereby certain cells acquired a special sensitivity to light ; 

 as would be expected, they were ectodermal cells initially developed in 

 the surface epithelium although on occasion they migrated below the 

 surface layer. Such cells may be found alone or may occur in associa- 

 tion with others to form an eye ; and in either case they may assume 

 several forms usually with well-defined characteristics, including a 

 specially sensitive receptor as well as an arrangement to conduct away 

 the excitation. Frequently the cell is associated with pigment which 

 serves as an absorbing agent, a fimction which becomes all the more 

 important when the sentient cells sink below the surface, in which case 

 the receptive pole is frequently surrounded by a pigment mantle 

 (Fig. 82). While thus aiding the receptor mechanism, an absorbing 

 pigment mantle is not essential and, indeed, is frequently absent. 



The light-sensitive cell usually assumes a specialized form which 

 may be differentiated into two main types. In the first and more 

 common type two poles are distinguished — a distal to receive the 

 stimulus of light and a jjroximal to conduct away the excitation. In 

 the fully developed bipolar cell, therefore, three regions are apparent : 

 a receptor or end-organ, a cell body with the nucleus, and a proximal 

 prolongation into a conducting fibre. 



The recejitive end-organ of the cell is often found to assume a 

 complicated form ; sometimes it is provided with digitations, presum- 

 ably in an attempt to increase the receptor surface (Fig. 87) ; more 

 frequently it undergoes specific modifications which can be classed as 

 belonging to two main types, cilia and rods, the second presumably a 



