§ 3.222 EFFECTORS WITH MOVABLE PIGMENT GRANULES 81 



Fig. 3-13. Two ommatidia from the compound eye of the prawn, 

 Palaemonetes , in longitudinal section to show pigment cells which 

 change shape, (a) In Z), the ommatidium is shown with the pigment 

 cells in the extreme dark-adapted position; in L, in the extreme 

 light-adapted position. As in Fig. 3-12, the pigment in the distal 

 retinal cells (d.r.c.) surrounds the cone (con.) in the dark position, 

 and the rhabdome (rh.) in the light. In L, the pigment in the proximal 

 retinal cells (p.r.c, not shown in D) has dispersed outwards from 

 below the basement membrane (just out of the figure), (b) The 

 same ommatidia with the pigment removed to show contractile 

 fibres (c.f.) in the distal retinal cells (d.r.c). They are attached to 

 a mass of accessory pigment (ac.p.). In D, the fibres are relaxed in 

 the dark-adapted position; in L, the fibres are contracted in the 

 light. Most of the cell protoplasm and the nuclei (d.r.n.) have moved 

 inwards with the pigment, leaving only an attenuated distal cell 

 process (d.p.). The nuclei of the proximal or retinula cells (r.t.n.) 

 remain stationary (from Welsh, 1930). 



diurnal rhythm that persists in constant darkness; but this is 

 reduced or absent in continuous light. The rhythm can be induced 

 experimentally to become out of phase with the time of solar 

 daylight. About one-third of the dark-adapting hormone of the 

 crab eyestalk is in the sinus gland, and two-thirds in the ''optic 



