145 



of the Rotifera. They include about 97 per cent, of the individuals 

 and almost all of the limnetic species. 



As a group they exhibit a seasonal routine which is a complex of 

 the records of individual species, and as such it reflects to a remark- 

 able degree a similarity to individual records, especially of the peren- 

 nial species. In general the Ploima are less abundant in colder 

 months, that is, below 50-60, than in the warmer ones from May to 

 October. Midwinter numbers are nevertheless considerable, 5,000 

 -35,000, and with the first rise of temperature in March we have, in 

 1898, a pulse of 175,000 which declines and again rises in a vernal 

 pulse of April-May, which vies with an autumnal pulse for rank as 

 the annual maximum. Following the vernal pulse there comes a series 

 of summer movements which vary from year to year. In 1898 they 

 grow smaller as the season wanes, rising again in September. In 1897 

 the autumnal pulse is the largest of the year and appears early in 

 September. In 1895, on the other hand, it is carried into the last 

 days of November. Numbers sink to the winter minimum shortly 

 after the winter temperatures are reached. In a general way the 

 direction of movement in the several parts of the seasonal curve of the 

 total Ploima is much like that of the individual species of which it is 

 composed. The differences lie in the amplitude of the pulses and in 

 slight changes in the locations of maxima and minima. There are, it 

 is true, many exceptions to this sweeping general statement, but it is, 

 nevertheless, both surprising and significant that the sum of so many 

 complex records should still preserve the recognizable outlines of its 

 parts. This is not due simply to the dominance of a few abundant 

 species, but is a combination of many, as will be seen frequently ^in 

 Table I., where species with insignificant numbers still show in their 

 seasonal occurrences some correlation with the movement of the 

 great mass of the totals. This similarity points to some common 

 factor in the environment common to all of the species. It is to be 

 found, I believe, in the food relations in the wax and wane of the food 

 supply. Most, if not all, ploiman rotifers are herbivorous, or at least 

 omnivorous, and find their food to a large extent in the phytoplankton. 

 I have already called attention to the recurrent pulses of the chloro- 

 phyll-bearing organisms. These primarily, but combined with other 

 and largely changing seasonal factors such as hydrograph and tem- 

 perature, are the basis upon which the superstructure of the seasonal 

 changes in the ploiman plankton are built. The correlation between 



