EUPAGURUS. 535 



that they are four in number. These cells are squamous 

 and tile-like, but their boundaries are very indistinct. 



(2) The crystalline cone (yit.) is composed of four 

 cells, and it extends from the corneal cells to the distal 

 end of the rhabdome. Each cone cell has a transparent 

 body tapering to its proximal end, which overlaps the 

 distal end of the rhabdome. The nuclei are at the distal 

 end of the cells. 



(3) There are two distal retinular cells (d. re.) at 

 opposite sides of the cone cells. They are deeply 

 pigmented and are contractile, their size varying with 

 the strength of light. 



(4) There are seven 'proximal retinular cells (j). re.), 

 and Parker has found that there is typically an eighth 

 which has become rudimentary. These cells surround 

 the rhabdome. Their distal ends — by the side of the 

 cones — are bulbous and contain the large nucleus, the 

 proximal end tapers off above the basement membrane. 

 The optic nerve fibrillae pass through the cells to end in 

 all probability in the " stiftchen " of the rhabdome 

 (fig. 33). They run up the outside of each retinular cell, 

 pass round the nucleus and return down the inner side. 

 It is this bundle of fibrillae round the nucleus which gives 

 the bulbous appearance to the distal end of the cells. 



The retinular cells are deeply pigmented from end 

 to end and thus form a complete dark sheath round the 

 rhabdome. Parker found that in the darkness the 

 pigment migrated completely into the retinal fibres 

 beneath the basement membrane, so that the rhabdome 

 became accessible to light from all sides. Similar 

 changes are induced by darkness in the distal retinular 

 cells (Iris cells), the pigment all retreating into the body 

 of the cell at its distal end. In daylight the pigment 

 extends down the proximal processes of the cells. 



