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 IX. -XII.) average about 75, 

 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), 55 per cent, and 6.5 per cent., respectively. 



This predominance of the spinous variety at high temperatures 

 is apparently a striking illustration from statistical evidence of the 

 hypothesis of Wesenberg-Lund ('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 1 1 ,400 belonged to amphiceros. The temperatures recorded 

 during this period ranged from 8-3 to 71. A second pulse started 

 October 5 at 1,600, culminated October 12 at 1,605,600, and declined 

 to on October 26. Of the total individuals (1,609,000) included in 

 this pulse, 894,800 belonged to amphiceros and 7 14,200 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 Brachionus pala. 



This variety is itself poly cyclic, as is evidenced by the recurrence 

 of male and winter eggs carried by the female at times of the pulses. 



