634 F. W. GAMBLE AND F. W. KKEBLE. 



and then passes away with the dawn. The time of the year, 

 the particalai' atmospheric conditions^ and the special nature 

 of the coast affect the time at which the full nocturnes 

 occur. Recovery is determined by the same factors. In 

 August and September we have noticed at Piel and at St. 

 Vaast that the change has usually taken place by 10 o'clock, 

 or in captivity by 11 p.m. ; while the resumption of the 

 diurnal colouring occurs at an early hour of the morning. In 

 winter (December) the times have been found to be 6 o'clock 

 at night and 7 in the morning, though on a clear early 

 morning in November (at dead low water) recovery may have 

 taken place by 5.30 to 6 a.m. 



If this conclusion be, as we believe, correct and new, then 

 it follows that the full appreciation of the causes underlying 

 colour-change in any particular case, is not possible until a 

 large body of experimental work has been gathered together 

 on this and analogous groups of reactions. 



The nocturnal condition, especially the antecedent greening, 

 may be induced artificially by exposing the prawns to 

 uniformly scattered light of low intensity. 



It may also arise towards evening in "light- induced 

 nocturnes ; " that is, in prawns which are exposed in por- 

 celain dishes to powerful incandescent gas-light. Nocturnes 

 artificially produced show marked loss of irritability. 



We have evidence that the nocturnal phase is a peculiar 

 state with respect to organs other than those of colour. The 

 muscular and connective tissues are much enhanced in trans- 

 parency, and the heart-beat is nearly twice as rapid (about 

 240 per minute) as it is during the day (150 per minute), 

 which facts point to a nervous condition very different from 

 that which obtains during the day. To establish these 

 changes in detail we require a much larger body of facts on 

 the normal nocturnal condition than we at present possess. 



The distinctive blue colour is only one of a number of 

 changes affecting the whole body, and it may prove to be the 

 least significant character of " nocturnes.'^ We have some 

 evidence that during the nocturnal phase the metabolism of 



