250 



by a fall to 5,120 on the 24th, and, in the rising temperatures which 

 then ensue, by a recovery to a second maximum of 11,000 on the 

 31st. Diaphanosoma then declines though temperatures continue 

 to rise. These fluctuations all take place in comparatively stable 

 hydrographic conditions. There is a suggestion in the records of 

 this year that rising temperatures in midsummer conditions tend 

 to accelerate, and falling temperatures to depress, development of 

 the Diaphanosoma pulse, and also that after the pulse has continued 

 for some time (six weeks in this instance) rise in temperature ceases 

 to be effective. The autumnal decline in Diaphanosoma may 

 therefore not always of necessity be due to temperature decline alone. 



In 1898 there are also two midsummer heat pulses, culminating 

 on July 26 at 89, and August 30 at 82.5, separated by a depression 

 which reaches 77 on August 16. The depression to 78.3 on August 

 2, with the consequent appearance of a third summit at 83 on 

 August 9, is due mainly to the fact that the temperature was taken 

 at 9:15 a. m., while all the others were in the late afternoon. The 

 seasonal curve of Diaphanosoma shows likewise two apices, the first 

 at 8,580 on July 26, and the second at 2,520 on August 30, separated 

 by a depression to 60 per m. 3 on August 16, when temperatures are 

 lowest. In this year the flood of the middle of August doubtless 

 plays a large part in depressing alike the thermograph and the 

 seasonal curve of Diaphanosoma, but in the light of the evidence 

 from 1897 in stable hydrographic conditions the direct influence 

 of temperature is also possible in this instance. 



Diaphanosoma is thus a late summer planktont w r hich in develop- 

 ment is very responsive to changes in temperature. It appears in 

 the plankton in small numbers shortly after the establishment of 

 summer temperatures in May- June, but does not begin its maximum 

 development until maximum summer temperatures have existed 

 for six to eight weeks, and is apparently incited to this by a summer 

 heat pulse. 



Males were recorded on July 18 and August 1, and ephippial 

 females on August 1 and September 5. Dead individuals were 

 most numerous during or subsequent to the maximum of the pulse. 



This species is reported by Apstein ('96) in the plankton of 

 Dobersdorfersee, where it is also monocyclic, first appearing in 

 May, and attaining its maximum in September, when the males 

 first appear. In contrast with conditions in our waters the maxima 



