183 



than 2 per cent ; and the form spinosus, less than 1 per cent. The 

 proportions formed by the several varieties fluctuate from year to 

 year and from season to season, indeed, from collection to collec- 

 tion (Table I.). Thus in the first three years pala exceeded am- 

 phiceros, while in the last two these conditions were reversed; 

 and in 1896 the form spinosus contributes 6.5 per cent, of the 

 individuals. The predominance of the pala-amphiceros group is, 

 however, preserved throughout all of the years. 



The species as a whole is found throughout the -entire seasonal 

 range of temperatures but with very great fluctuations in numbers. 

 Speaking generally, there are vernal and autumnal pulses separated 

 by a midwinter minimum which is well sustained, developments in 

 excess of. 5 ,000 per m 3 being very rare in this season. There is also 

 a midsummer minimum more or less diversified by pulses of some 

 magnitude. This sequence was not fully realized in any single year 

 of our records, but this may be due in part to insufficient collections 

 at times of the major pulses. Thus in 1894 only a small autumnal 

 pulse (13,650) was detected. In 1895, there was a small vernal 

 pulse (67,338), and a belated autumnal pulse (320,915) lasting a full 

 month in November-December. In 1896, there was a very abrupt 

 vernal pulse rising from 53,618 on April 17 to 1,012,350 on April 24, 

 while in the fortnightly fall collections the only pulse detected was 

 one of 14,000. In 1897, the monthly collections of the spring seem 

 to have missed all considerable developments, the largest recorded 

 being only 16,000. On August 31 and October 12 of that year, 

 however, there were pulses of 1,398,000 and 1,605,600. In 1898 

 there was a well-developed vernal pulse of 451,200 and a small 

 autumnal one of 83,200. 



The species is not, however, dicyclic, for both the winter and 

 summer interims are marked by occasional recurrent pulses of 

 smaller proportions. The table on the next page shows the loca- 

 tions and temperatures of the culminations of these pulses. 



From this table it is evident that a wide range of optimum tem- 

 peratures is possible. Nevertheless, 23 of the 31 pulses occur above 

 50, and 21 of them above 60. In 1898 only 3 per cent, of the 

 individuals are found below 57, and with the exception of 1895 

 approximately these conditions will be found in the other years. 

 Brachionus pala is thus a perennial planktont, but as a rule it reaches 

 its largest developments only above 60 in our channel waters. 



