638 F. W. GAMBLE AND F. "W. KEEBLE. 



is especially well shown by the brown specimens in " water- 

 circulator" B and those in Flask Aa (pp. 686, 687). 



Other experiments with the same object show that the 

 longer continuous darkness is employed, the more it seems to 

 wear down the periodicity ; so that, after a few days, noc- 

 turnes may be found at almost any hour of the morning. 

 Moreover an extraordinarily irritable condition supervenes, in 

 which Hippolyte rapidly responds to alternations of light 

 and darkness. Thus (Table I, specimen A in Flask A) a 

 large specimen, which had been under observation since 

 July 29th, and in the dark most of the time, at 7.15 a.m., 

 August 2nd, had recovered slightly to a vivid transparent 

 green, and after two and a half hours' exposure to diffuse 

 light, had become brown. It was now covered again, and at 

 2 o'clock was already almost fully nocturnal, and was then 

 exposed to light for half an hour, and once more regained 

 its brown colour though not fully. It was now put in the 

 dark, and at 4.15 was a good nocturne, but lost much of its 

 blue transparency during ten minutes' exposure to light. 



Whether more prolonged, more complete exposure to dark- 

 ness would result in the formation of permanently nocturnal 

 Hippolyte we have not determined, but experiments lasting 

 twenty-four hours, eighteen, and thirty-six days, made by 

 Professors Brooks and Herrick, on Palsemonetes varians 

 (1895), showed that in all three cases when the dark chamber 

 was unsealed the prawns originally of a shade of light brown 

 or brownish green had become nearly white and looked 

 bleached. Unfortunately these authors give no information 

 as to whether recovery took place, their object being to test 

 the effect of darkness on the colour of the newly hatched 

 larvse, and on the pigment of the eyes of the young and of 

 the mother. They found that there was no appreciable dif- 

 ference between the colour or the optic pigment of these 

 larv99 and of others born under natural conditions, but that 

 in the eye of the parent prawn continued darkness had pro- 

 duced a remarkable migration of the pigment and distal 

 retinular cells outwards towa^rds the cornea. 



