SENSORY ORGANS AND RECEPTION 



317 



persistent periodicity in retinal pigment migration, observed in many 

 crustaceans, depends upon regular rhythmicity in the release of the 

 regulatory hormones. In Palaemonetes the production of the dark- 

 adapting hormone reaches its maximum about midnight, and is minimal 

 at dawn; during the daylight hours the hormone is stored in preparation 

 for the nocturnal phase of the cycle. 



In Palaemonetes the eye-stalks are the chief source of the retinal pig- 

 ment-dispersing hormone, but lesser amounts occur elsewhere in the 

 c.n.s. (brain, connectives and ventral ganglia). Similarly, stores of dis- 



0-20 







60 



240 



120 

 Time (minutes) 



Fig. 8.10. Response of the Distal Retinal Pigment of Dark- adapted 

 Prawns (Palaemonetes) to Injection of Eye-stalk Extracts 



Maximal dispersion of pigment (light-adaptation) occurs 30-60 min after injection., 

 (From Brown et al. (13).) 



persing hormone have been found in the brain, optic ganglia and other 

 nervous centres of grapsoid crabs (10, 11, 13, 14, 83, 84, 89, 90, 90#, 130, 

 135, 142, 151). 



Photomechanical Changes in Fishes. In fishes three mechanisms are 

 concerned in regulating the amount of light which reaches the retina, 

 namely alteration of pupillary aperture, migration of retinal pigment and 

 movement of visual cells. These adaptations are found in diurnal species 

 of the neritic zone. 



Pupillary Movement. According to their habits, three groups of 

 selachians can be distinguished. These are diurnal (day-feeding) sela- 

 chians such as Mustelus, having pupils wide open in the daytime ; nocturnal 

 (night-feeding) forms, e.g. Scyliorhinus, Raja, whose pupils close almost 

 completely in daylight; and deep-sea forms such as Spinax, with large eyes 

 having wide pupils and weak iris musculature. The sphincter iris muscle 



