183 
than 2 per cent; and the form spznosus, 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 spimosus contributes 6.5 per cent. of the 
individuals. The predominance of the pala-amphiceros group 1s, 
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 1n 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® 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. 
