316 



THE BIOLOGY OF MARINE ANIMALS 



One of these substances is a pigment-dispersing principle or hormone. 

 When extracts prepared from the eye-stalks of light-adapted prawns 

 (Palaemonetes) are injected into dark-adapted animals retained in darkness, 

 the distal and reflecting eye-pigments move proximally into the positions 

 characteristic of the light-adapted state (Fig. 8.10). Eye-stalk extracts 

 taken from light-adapted animals are more effective in causing pigment 

 migration than extracts prepared from dark-adapted specimens (Fig. 

 8.6). This result indicates a higher content of pigment-dispersing hormone 

 in the sinus glands of light-adapted animals (vide p. 445). A pigment- 



NMNMNMNM NMNMNMNMN 

 13 14 15 16 11 18 19 20 21 



M NMNMNMNMNMNMNMNMN 

 22 23 24 25 26 21 28 29 30 



Fig. 8.9. Diurnal Movements of Eye-pigments of a 

 Nocturnal Shrimp Anchistioides antiguensis 

 Records of the movements of distal pigment cells of four animals kept in constant 

 illumination (hollow circles), except for the last two days, and of four animals kept in con- 

 stant darkness (solid circles). N, M, noon and midnight. In the sketch of an ommatidium 

 (left), A is the extreme peripheral or night position of the distal pigment cells ; B, the 

 inner or day position, in constant darkness. Plotted points refer to distance from 

 cornea to outer boundary of the pigments (each unit = 10 pi). (From Welsh (154).) 



dispersing principle has also been recognized in the eye-stalks of many 

 brachyurans (Cancer, Uca, Libinia, etc.). 



Even a brief exposure to light (5 min for Palaemonetes) suffices to release 

 enough stored dispersing-hormone to produce light-adaptation of the 

 distal pigment cells. Light-adaptation continues for a further 10 min 

 after the animal is returned to darkness, indicating continued release of 

 dispersing-hormone during that period, and its accumulation in the blood. 

 Other work points to the existence of a second hormonal factor involved 

 in dark-adaptation. 



It appears that some controlling centre is activated by photic stimula- 

 tion, and the degree of activation, in turn, determines the relative pro- 

 portions of the two retinal pigment hormones which are secreted. The 



