SENSORY ORGANS AND RECEPTION 



315 



ommatidia as well (superpositional eye) (Fig. 8.6). The reflecting white 

 pigment bordering the retinular cells migrates proximally beneath the 

 basement membrane in daylight, and extends distally about the retinular 

 elements in darkness. In the latter arrangement it forms a functional 

 tapetum or reflecting layer (Palaemonetes). The positional changes which 

 the retinal pigments undergo show much variation in different species. 

 In Palaemonetes, as noted, all three pigments migrate, whereas migration 

 is limited to the proximal retinal pigment in the lobster Homarus (86, 90, 

 112, 118). 

 In addition to pigment movements evoked directly by changes in 



At day 



Light 



c. he. 



DarU 



At night At day 



At nigbt 



Fig. 8.8. Influence of Diurnal Rhythm on the Position of Iris (Retinal) 

 Pigments in the Compound Eye of a Decapod Crustacean 



(a) Shows the pigment systems in light-adaptation during the day and during the 

 night, respectively; (b) shows the migration of the pigment in the dark-adapted eye 

 under the influence of the diurnal rhythm in daytime and at night, c, cornea; h.c, 

 hypodermis cell; cr.c, crystal cone; i.t., iris tapetum; i.p., iris pigment; rh., rhabdome; 

 r.p., retinal pigment; m.f., membrana fenestra; t., tapetum; e.s., eye-stalk. (From 

 Henkes (68).) 



environmental illumination, there are rhythmically occurring diurnal 

 migrations which take place independently of any changes in light in- 

 tensity, and persist even when environmental conditions remain constant 

 (Figs. 8.8, 8.9). The occurrence of persistent diurnal rhythms in the 

 migration of eye pigments has been noted in many Crustacea (Palaemonetes, 

 Portunus, Homarus, etc.). In Palaemonetes the diurnal rhythm continues 

 for months in animals which are kept under conditions of constant 

 darkness and temperature. Persistent rhythmical movements of retinal 

 pigment can also be observed under conditions of constant illumination, 

 but are of much smaller magnitude (68, 85, 113, 130, 154, 155). 



It has been discovered that the eye-stalks of various decapod crustaceans 

 contain substances capable of influencing the position of the eye-pigments. 



