654 THE EYE IN EVOLUTION 



results attributable to the activities of the endocrine system were initially demonstrated 

 by experiments involving the implantation or excision of this gland (Perkins, 1928 ; 

 Perkins and Snook, 1932 ; Hanstrom, 1933-40 ; Welsh, 1941 ; Kleinholz, 1942 ; 

 Brown, 1940-48). Later experiments on several species, however, showed that 

 although these effects were frequently dramatic if the entire eye-stalk were removed, 

 they were merely partial or temporary if this gland alone were carefully excised 

 (Kleinholz, 1948-49 ; Havel and Kleinholz, 1951 ; Travis, 1951 ; Welsh, 1951 ; 

 Passano, 1951-52 ; Bliss, 1951-53). Svibsequent histological investigation with the 

 appropriate technique demonstrated that this strvicture represented a gland-like 

 accumulation of enlarged nerve-endings associated with the axons of neuro -secretory 

 cells located in the x-organ and elsewhere in the eye-stalks and cerebral ganglion, 

 indicating that the real role of the sinus gland is a storage -release centre of the colloid- 

 like secretion of the cells of the neuro -secretory system (Bliss and Welsh, 1952 ; 

 Carlisle, 1953 ; Bliss et al., 1954). 



The functions of the hormones secreted by the neuro -endocrme system 

 of Crustaceans are complex ; those of greatest interest to us concern the 

 integumentary and retinal pigmentation. In most cases there is no precise 

 knowledge of the nature of these hormones or the site of their elaboration 

 within the many ganglionic masses comprising the system. The integumen- 

 tary chromatophores are regulated by three or four different chromato- 

 PHOROTROPINS, soHie of wliich determine the concentration of pigment, 

 others its dispersal. These have already been discussed ^ but it may be 

 useful to recapitulate here that the release of these hormones is regulated 

 by the degree of illumination and the nature of the background ; the 

 receptor organs are the retinse, differential stimulation of the dorsal or 

 ventral areas of which may determine the release of different hormones so 

 that adaptation to the background is attained. 



In addition to these environmental variations, we have already seen ^ 

 that in many species a diurnal rhythmic release of the hormones causes a 

 dispersal of pigment by day and its concentration by night, a habit which 

 tends to persist in spite of artificial disturbances of the natural day-night 

 sequence ; this rhythmic behaviour is an acquirement of the neuro -secretory 

 centres (Roller, 1925-30 ; Perkins, 1928 ; Brown, 1940-46 ; and others). 



Betinal 2iigi'nentation is under the control of at least two chromato- 

 phorotropins of an unknown chemical nature different from those responsible 

 for changes in the colour of the integument, one regulating pigment migration 

 in the dark, the other in the light. Here again, illumination and background 

 are the determining factors rn the release of the hormones and the effect is 

 abolished if the optic nerve is cut (Smith, 1948 ; Sandeen and Brown, 

 1951-52) ; the hormone regulating pigmentary migration in the dark is often 

 liberated in a persistent diurnal rhythm which gives a basic 24-hour variation 

 to this activity also ^ (Welsh, 1939-41 ; Brown, 1951 ; Brown et al, 1951). 

 In the prawn, Leander, it appears that migration of the distal retinal 

 pigment depends on the hormones of the sinus gland modified by illumination, 



1 p. 93. 2 p i9_ 3 p, 19. 



