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 Plowma 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 againin 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 Plowma 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 
