190 
ceros forms of this group in the period from June 1 to October 1 is 
from 86 to 96 per cent., averaging 93.5 per cent. in the several years. 
On the other hand, in the colder months—Jan. 1 to June 1 and-Oct. 1 
to June 1—the per cent. is only from 13 to.83, averaging 45. The 
temperatures on June 1-(Pt. I, Pl. EX—XI1.) average about aa 
and on Oct. 1 about 67°. The spinous form (amphiceros) thus in- 
cludes about 45 per cent. of the individuals at low temperatures, and 
93.5 per cent. at high temperatures; and the smoother form (pala 
type), 05 per cent, atid 6:5 per cent. respectivelys 
This predominance of the spinous variety at high temperatures 
1s apparently a striking illustration from statistical evidence of the 
hypothesis of Wesenberg-Liund (’00) that such elongations of the 
body of planktonts are adaptations to the lessened buoyancy of the 
warmer water. This relation of the spinous form to higher tem- 
peratures is evident in every year, 1895-1898, and the proportion 
of spinous forms, 86-96 per cent., exhibits all the constancy that 
might at the best be expected in plankton data. The relation is 
generally apparent (Table I.) in the individual entries as well as in 
the sums total, and, considering the numbers concerned and the 
long period of observation, should have more weight than some of 
the exceptions to the hypothesis, which have been or will be noted, 
in which the data are less extensive. For example, Brachionus 
pala var. dorcas does not in its seasonal distribution support the 
hypothesis, but owing to its small numbers—especially of the form 
Spinosus—less weight should attach to its evidence. 
In 1897 the first autumnal pulse of the pala group consisted 
almost entirely of var. amphiceros. This pulse started August 10 
at 3,600, culminated August 31 at 1,398,000, and declined to 800 
September 29. Of the 3,500,200 individuals included in this pulse, 
all but 11,400 belonged to amphiceros. The temperatures recorded ~ 
during this period ranged from 83° to 71°. A second pulse started 
October 5 at 1,600, culminated October 12 at 1,605,600, and declined 
to 0 on October 26. Of the total individuals (1,609,000) included in 
this pulse, 894,800 belonged to amphiceros and 714,200 to pala. The 
range in recorded temperatures in this period was from 71° to 59.5°. 
This may serve as an additional illustration of the relation of tem- 
perature to the spinous variety of Brachtonus pala. 
This variety is itself polycyclic, as is evidenced by the recurrence 
of male and winter eggs carried by the female at times of the pulses. 
