306 Birge — The Crustacea of the Plankton. 



It will be seen that in 1895 there were present only three 

 species, while in 1896 two others were added. In 1897 the con- 

 ditions were essentially similar to those of 1895. Indeed, while 

 the time from which my observations have extended by no means 

 warrants any positive assertion in the matter, there seem to be 

 distinct indications of a biennial periodicity in the plankton in 

 respect to Crustacea, algae, and rotifers. Observations must be 

 continued, however, over a much longer time before any definite 

 statement can be made on this subject. 



The winter numbers of each species are on the whole singu- 

 larly constant through the season, as will be seen by reference 

 to the tables giving the numbers of the several species. The 

 death rate must be very low. During the period, January- 

 March, the variation in the number of Crustacea taken in 

 twenty or more catches made each winter vary to an extent 

 hardly greater than might be found in catches made close to- 

 gether on the same day. It would be very difficult to prove 

 any considerable decline in numbers of Diaptomus or Daphnia 

 during the winter and they do not increase by reproduction. 

 Cyclops produces eggs much more abundantly than the other 

 species, and the adults seem to become fewer in late winter and 

 late spring, but their number is more than made good by young 

 individuals. In 1895 Cyclops began to show numerous egg clus- 

 ters in February, and about ten per cent, of the specimens were 

 egg-bearing females. These eggs developed very slowly, and 

 few nauplii and almost no young Cyclops were seen. In 1896 

 the reproduction of the Cyclops hardly stopped at all during- 

 winter. In the middle of January nearly one-half the Cyclops 

 bore eggs, and numerous nauplii were present. By the middle 

 of March the nauplii had grown to young Cyclops, from three- 

 fourths to seven-eighths of the total number of the species were 

 immature young. 



The winter minimum therefore falls in the period before 

 Cyclops has begun this winter reproduction. In 1895 the mini- 

 mum came in January and in February in 1896. Yet through- 

 out the winter months the numbers are so constant that no well 

 marked minimum can be placed at any date. In 1897 the condi- 

 tion of Cyclops was intermediate between those of 1895 and 1896. 



