HIPPOLYTE VAETANS. 639 



Effect of Constant Illumination (Table VIII, p. 683, 

 and Summaiy, p. 652). — A series of "water-circulator" 

 glass vessels covered with muslin were placed in porcelain 

 dishes against a background of white cardboard. The cur- 

 rent of water circulating through the vessels kept the 

 temperature low and fairly constant. The vessels were 

 illuminated by incandescent light, but a source of error was 

 admitted by our inability to screen the vessels from diffuse 

 daylight. An experiment was commenced on September 3rd 

 with six brown, yellowish-green, and red specimens. The 

 lamp was lit at 5 o'clock, and at 9 p.m. all six were nocturnes. 

 The next day at noon the prawns had recovered to an emerald 

 green colour, which was maintained till 6 o'clock, and at 

 9.50 p.m. had passed again into the nocturnal blue tint. 

 The next day (September 5th) at 11 a.m. the prawns were 

 green, but in two cases had recovered in the tail region to a 

 grey or greyish-brown colour. At 7.30 p.m. two were noc- 

 turnes, two were sick and moulting (a constant source of loss of 

 specimens), two were transparent green (Table VIII, p. 683). 

 In another experiment (made on August 26th and 27th) the 

 diurnal phase continued during the times of observation, but 

 the majority of observations on the effect of incandescent 

 light at night (as was pointed out in another connection on 

 p. 629) tend uniformly to the conclusion that it does not 

 inhibit the recurrence of the nocturnal phase, at any rate for 

 the first two nights; though here again we do not know the 

 effect of long-continued illumination which occurs in nature, 

 for example during the summer on arctic shores. 



It might be urged, however, in regard to these experi- 

 ments, that the occurrence of nocturnes should not be con- 

 sidered as evidence of periodicity (which is our view), but as 

 indicating the power of response of Hippolyte to a very 

 small difference of light intensity. For, during the day, the 

 observation dishes are exposed to a certain amount of diffuse 

 light (reduced as far as possible by drawn blinds) in addition 

 to incandescent light, while, during the night, the latter is 

 alone operative. This objection may be met by an appeal to 



