22, TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
light up until it has been kept in the dark for half an hour, and that the 
intensity is not at the maximum for another additional half-hour. 
The following passage from Massart describes the variations as 
observed in Noctiluca :— 
‘The experiments show that the irritability is dependent on the 
alternations of day and night, the Noctiluca is hardly excitable on shaking 
during the day and shines only during the night. Fact still more curious, 
whether the organisms are submitted to the alternations of day and night, 
or whether they are maintained in constant illumination or constant 
obscurity, they still remain much more excitable during the night than 
during the day. It is a veritable phenomenon of memory, everything 
looks as if the Noctilucae preserved the recollection of the regular 
succession of the days and nights.’ 
Massart compares this to the change in position of the leaves of plants — 
during day and night in the Oxalis and certain Papilionaceae, but adds 
that while the phenomenon lasts only some days in plants, in the 
Noctilucae it lasts until the death of the animal. 
His experiments at the outside limit, however, lasted for one week 
only, when the organisms died; in the present set of observations the 
diurnal alternation of activity was followed with organisms kept in 
continuous darkness for twelve days, and although the number of living 
organisms was decreasing all the period, a few were still left alive and 
phosphorescent at night at the end of the pericd. 
Since the fact of this diurnal periodicity is one of the most striking of 
those alternating habits or functions of the lower invertebrates which bear 
such a curious resemblance to memory in higher vertebrates, and, indeed, 
have been regarded as a rudimentary memory,' it may be regarded as 
sufficiently interesting to merit a detailed description. It appears to stand 
in some danger of being forgotten, since it is not mentioned even in the 
larger of the modern text-books, and to the best of my knowledge it has 
not been shown to exist in the phosphorescent copepoda, nor demonstrated 
as persisting for such a long period as in the present experiments. Also 
its onset at the close of the day and gradual extinction at dawn have not 
previously been followed with any exactitude. 
1. See F. Darwin, Presidential Address, Brit. Association, Dublin, 1908. 
