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scarcely affect the urchins, for they are not sessile animals, but 

 move actively and their vertical range of migration during the 

 course of an hour is often far in excess of this figure. 



The possibility of tidal influence could be tested by keeping 

 urchins in a floating cage. If the lunar reproductive cycle 

 were thereby abolished the tidal connection would be demon- 

 strated ; but a contrary result from the experiment would not 

 dispose of a possible influence of the tides, for an established 

 rhythm in a physiological process is often persistent after the 

 original cause has been removed. One has only to recall the 

 case of Convolitta, a marine turbellarian worm which lays its 

 eggs only at neap tides. It has been found that animals hatched 

 and reared in the laboratory keep the same egg-laying habit 

 (Gamble and Keeble, Quart. Journ. Micr. Soc, 1906). In any 

 case the experiment of eliminating the tides was unfortunately 

 impracticable with Diadema owing to its size. Fully grown 

 specimens measure over one foot from tip to tip of the spines, 

 and it was impossible to obtain large enough floating boxes to 

 contain a hundred or more individuals. But I intend to seek 

 further evidence regarding the possible effect of tides by study- 

 ing Echinoids in localities with greater and lesser tidal ranges 

 than at Suez. I am convinced, however, that if a similar lunar 

 reproductive cycle exists in the sea-urchins at Naples or 

 Plymouth it is very little pronounced. For I have made use 

 of the Echinoids at these places to obtain spermatozoa and ova 

 for other experimental purposes for months continuously without 

 ever noticing a rhythmic variation in condition or quantity of 

 genital products. At Suez the period in each lunar month 

 when spermatozoa and eggs are unobtainable would necessarily 

 force itself upon the notice of the investigator. 



The possibility of a direct effect of the light of the moon 

 on the Echinoids could be tested by keeping specimens in the 

 dark. Although the large size of Diadema again precluded 

 this experiment at Suez, I intend to carry it out in another 

 place with a smaller Echinoid. If the light has an effect it 

 must necessarily be more constant in the cloudless summer 

 nights of Egypt than in Europe. 



Another possibility was that the light of the moon might 

 act by causing the urchins to feed either more or less on moonlit 

 nights. A systematic examination and comparison of the 

 gut-contents of specimens taken at dawn (i) after a moonlit 

 night, and (2) after a night without moon, showed, however, 

 no difference either in quantity or nature of food. 



The sea-urchins fished at Alexandria are locally believed to 

 change with the moon in the same manner as the Suez ones. 

 A parallel investigation of the Alexandrian urchins (Strongylo- 

 centrotiis lividus) has shown, however, that in this case there 



