io8 



THE PINEAL ORGAN 



epithelium, the cells of the latter having sunk down into the subepithelial 

 tissue, where they develop dendritic- and axonal-processes, thus becoming 

 bipolar. These cells which retain at first their direct connection with the 

 surface are called the primary receptor cells. Their place on the surface 



Fig. 69. 

 A — Limulus polyphemns (dorsal aspect). 

 B — Larval form of Limulus (Trilobite stage). 

 C — Prestwichia rotunda. 



The position of the paired median and the paired lateral eyes is indicated. 

 (After H. Woodward.) 



is afterwards taken by the formation of new cells which later also send 

 processes into the subepithelial tissue. The primary receptor cells now 

 losing their connection with the surface are termed secondary or bipolar 



ant'— \ 



Fig. 70. — The Left Side of Cypris, exposed by the Removal of the Left 

 Valve of the Shell, showing the Median Eye and Appendages. (After 

 Gerstaecker and Zenker.) 



neurones. By a repetition of the process a third layer of cells is formed, 

 so that eventually the retina consists of three principal layers of cells with 

 their communicating processes (Fig. 12, Chap. 3, p. 17). Such a retina 

 is known as a compound retina, and is comparable in this respect with the 



