LUNAR PERIODICITY IN LIVING ORGANISMS 275 



molluscs, and crabs is stated to vary with the phases of the 

 moon : the animals are said to be " full " when the moon is 

 full, and empty at new moon. Some gourmets maintain too 

 that the taste varies with the moon. 



With the object of testing the truth of the popular state- 

 ment I made systematic examinations of the testes and ovaries 

 of a sea-urchin [Diadema setosum) at Suez during the summers 

 of 1920 and 1 92 1. For this species of Echinoid the reports 

 of the fishermen turned out to be founded on fact to a surprising 

 degree. There is a periodic reproductive cycle correlated with 

 the lunar period, the genital products being spawned into the 

 sea round about each full moon during the breeding season. 

 After spawning the testes and ovaries become reduced in bulk, 

 the declining phases of the moon being occupied by recupera- 

 tion and the commencement of the preparation of fresh genital 

 products. After the new moon these increase in ripeness and 

 in bulk, and with this development the visible size of the genital 

 glands grows once more. 



Microscopic examination of the gonads of a number of 

 individuals between the first quarter and full moon shows the 

 majority to be swollen and packed with fully formed sper- 

 matozoa or eggs. A small proportion are " spent," that is they 

 show evidence of having lately extruded their genital products. 

 A week later the relative proportions are reversed ; some 

 individuals have gonads still full of spermatozoa or eggs, but 

 most are " spent." Between the third quarter and the new 

 moon all gonads are shrunken in size and contain nothing but 

 developing spermatocytes or oocytes. From now onwards 

 until the first quarter of the next moon these cells show pro- 

 gressive stages in development into spermatozoa and eggs, 

 destined to be spawned round about the time of full moon. 

 This lunar cycle is repeated throughout the breeding season. 



In seeking a causal connection between the reproductive 

 rhythm and the lunar month an influence of the tides first 

 suggests itself. But whereas Diadema has a single reproductive 

 cycle in each lunation there are two spring and two neap tidal 

 periods, i.e. a double cycle. However, during the summer 

 months at Suez the new moon spring tides have a greater range 

 than the full moon springs, so that the maximum tidal range 

 is attained once only during each lunar month. The higher 

 and lower water at the new moon springs might conceivably 

 react on the Echinoids by the different hydrostatic pressure 

 (affecting, e.g., the tension of dissolved gases) or by causing the 

 animals to be at a greater or lesser distance than usual from 

 the source of oxygen or of light. But the average excess tidal 

 range at new moon springs over that at full moon springs during 

 the period studied was only 58 cm. This small difference could 



