149 



A. aculeata var. valga Ehrbg. was seen frequently. A. serrulata 

 Ehrbg., regarded by Weber ('98) as a variety of A. aculeata, was 

 recorded Jan. 24, 1899, and found by Hempel ('99) in December. It 

 seems to be rare in our plankton. Forms approaching A. aculeata 

 var. brevispina Gosse were also noted, but they, too, are rare, being 

 recorded only in February and March, 1899. A. aculeata var. 

 curvicornis Ehrbg. was noted April 29, 1896, at 70. 



Anurcea cochlearis Gosse. Average number, 69,393, distributed 

 as follows: A. cochlearis (sensu strictu) together with A. cochlearis 

 var. macracantha Lauterborn, 9,421; A. cochlearis var. tecta Gosse, 

 15,432; and forms with posterior spine of intermediate length 

 between cochlearis and tecta which include A. cochlearis var. stipitata 

 Ehrbg., 44,540. Numerically this is one of our important species, 

 containing over one ninth of all the rotifers in 1898. It is surpassed 

 only by Brachionus bakeri (with varieties included) , Polyarthra, and 

 SynchcBta. Average number of eggs, 32,358. 



This is a perennial planktont, appearing in every month of the 

 year throughout the whole range of temperature. Its entire absence 

 in August, 1898 (Table I.), is not paralleled in any other year. In 

 1897, for example, there is a well-developed pulse of 45,600 on 

 August 24. In 1894, 1895 , and 1896 there is a midsummer minimum 

 of a few weeks' duration in July, August, or September, but it is 

 irregular in its location. 



While the appearance of sexual cycles was not traced by the 

 records of males and winter eggs, a matter of some difficulty and 

 uncertainty in preserved plankton material, the existence of such 

 cycles is suggested by the recurrent pulses of occurrence in this 

 species (Table I.). It is possible that the species is poly cyclic in 

 our waters. The pulses in 1898 are well defined, in fact, somewhat 

 better than in previous years. The following table gives the num- 

 bers in the pulses in the several years and the dates and tempera- 

 tures at which the maxima occurred. 



All of the large pulses save those of November and December 

 and one at the close of October (Oct. 25, 1898, 28,500) lie at tempera- 

 tures above 60. The vernal pulse of April-May is the largest and 

 appears between 60 and 70, and the amplitude diminishes as the 

 period of maximum heat progresses, though in 1898 there was a 

 recurrence of larger numbers as temperatures fell. The optimum 



