138 
of total oligocheetes. These records throw some light on the condi- 
tions controlling the occurrence of oligochzetes 1n the plankton and 
their seasonal distribution. é 
They occur in all months of the year and throughout the whole 
seasonal range of temperatures. They appear in the plankton most 
frequently and in largest numbers in disturbed hydrographic condi- 
tions. Thus, of the 31 collections made in 1897, only 6 contained 
oligochztes, and the average number per m.* was only 32. Five of 
the 6 collections containing oligocheetes were made during the run-off 
of flood waters from impounding backwaters. In 1898, a year of 
much disturbed hydrograph (Part I., Pl. XII.), there were 52 col- 
lections, in 35 of which oligocheetes occurred with an average number 
of 76 per m.* Over 50 per cent. of the non-occurrences of oligo- 
cheetes fall in the more stable conditions of January, July—August, 
and December. The seasons of run-off from impounded backwaters 
are in all years favorable to the occurrence of oligochztes in the 
plankton. This is in sharp contrast with the nematodes, which 
appear with rising floods and access of tributary waters. The 
oligochetes are thus largely adventitious, at times when run-off 
from vegetation-rich backwaters prevails, and when Lemnacee and 
Ceratophyllum are washed into the channel by hydrographic changes. 
ROWE ARVAT: 
(Plates III. and IV.) 
Average number, 592,416, of which 195,326, or 33 per cent., are 
eggs, free or carried externally by the parent. Records were kept 
of males, of females, of females with eggs, of attached and free, 
summer, winter and male eggs, and of parasitized and dead indi- 
viduals. 
Rotifers occur in every collection and at all seasons of the year. 
Numbers are uniformly low (below 75,000 per m.* and often below 
15,000) during minimum temperatures from late in December till 
early in March. At other seasons of the year numbers fluctuate 
greatly, rarely reaching the level of the winter minimum except 
occasionally at the depressions between pulses. The curve of 
seasonal occurrence falls into the form of recurrent pulses (PI. III. 
and IV.) previously noted for other organisms. Of these pulses the 
vernal one in April-May is uniformly high, attaining 3,954,920 per 
