LIGHT AND METABOLISM 19 



secreted in the eye -stalks and central nervous system and stored in the 

 sinus glands and these regulate ovarian maturation and testicular 

 development. In Vertebrates the pituitary gland exerts an analogous 

 gonadotropic influence under the control of its associated centres in 

 the diencephalon which in turn receive their stimulation from the 

 retinae. 1 



PHOTOPERiODiSM IN PIGMENT MIGRATION. Pigment, the fuuction 

 of which is so closely related to light, would be expected to be peculiarly 

 susceptible to its influence ; in its migration to cause colour changes,^ 

 rhjrthmic diurnal variations of a primitive type frequently survive. 

 That a persistent rh}i:hm of this kind occurs in the migration of the 

 retinal pigment ^ in the eyes of a noctuid moth, Phisia garmna, was first 

 reported by Kiesel (1894), an observation which has been repeated in 

 several Arthropods with compound eyes and shown to persist even 

 although the animals are kept for a considerable time in conditions of 

 constant illumination and temperature or are reared from the larval 

 stage in the laboratory in constant darkness.* The effect is well seen 

 in the crayfish in the eye of which there is a tapetal reflecting pigment ^ 

 obscured during the day but unprotected at night so that the eye 

 then assumes an orange glow ; even if the animal is kept in conditions 

 of constant darkness and temj^erature, the diurnal rhythm of orange 

 " eye-shine " at night will continue automatically for months (Welsh, 

 1941). Similar rh}i;hms affecting the retinal and tapetal pigments are 

 seen in many species of Crustaceans (Henkes, 1952), and it would 

 appear that these pigmentary movements are under hormonal control, 

 a subject which will be discussed in a later chapter.^ 



Closely associated with the movements of the retinal pigment are the 

 corresponding movements of the rods ayid cones of some of the lower Vertebrates. 

 As with the retinal pigment, these movements are usually a direct response to 

 light, but evidence was produced by Welsh and Osborn (1937) that these elements 

 in the eye of the catfish underwent a diui-nal rhythmic change of position even 

 although the fish were kept in constant darkness ; the mechanism of this 

 rhythmic activity is unknown. 



The integumentary cliromatopliore sy stein frequently shows similar 

 cyclic activities. The responses of this pigmentary system to light are 

 complex and will be studied in a laier section ^ ; it is sufficient to note 

 here that many animals show a rhji;hmic day-night change of colour 

 wherein they pale by night and darken by day, a rhythm which may 

 persist for a considerable time if they are kept in conditions of constant 



1 p. 556. 2 p_ go. 3 p_ 170. 



* In the beetle, Bolitotherus cornutus — Park and Keller (1932) ; and a number 

 of Crustaceans such as fresh-water shrimps, Macrohrachium and others — Welsh (1930) ; 

 crayfish, Cam6a rws—Bennitt (1932), Welsh (1939-41) ; crab, C7ca— Smith (1948); 

 Brown et al. (1951-54), Kleinholz (1937) ; and so on. 



6 p. 165. « p. 547. ' p. 82. 



