14 



esting to determine whether the use of stronger light begun 

 as soon as daylight commences to wane would result in com- 

 plete inhibition of the process. 



It is evident from the above statements that Gonio - 

 nema is exceedingly sensitive to external conditions. Not 

 all coelenterates are affected in the same degree, some are 

 apparently not affected at all, by changes in illumination. 

 Some medusae lay their eggs always in the early morning, 

 while others of nearly related genera choose the evening or 

 night (Brooks - Life History of the Hydromedusae , p. 399). 

 Experiments carried on by Wilson and Donaldson at Beaufort 

 under Professor Brooks' direction (loc.cit.) showed that in 

 the case of Renilla and some sea-anemones, at any rate, 

 changes in light and temperature did not affect the precis- 

 ion with v^hich the regular physiological activities took 

 place. It is well known that a great many marine animals 

 show more or less definiteness in the habit of spawning. 

 Metschnikoff gives a table on page 25 of his Embryological 

 Studies showing the time of spawning of a large number of 

 medusae. In other groups the same tendency is manifest. 

 This phenomenon is probably the result of the working of 

 natural selection, the habit of laying the eggs at a cer- 

 tain definite time having proved of value to the different 



