138 



of total oligochsetes. These records throw some light on the condi- 

 tions controlling the occurrence of oligochastes in the plankton and 

 their seasonal distribution. 



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 

 oligochastes, and the average number per m. 3 was only 32. Five of 

 the 6 collections containing oligochaetes 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 oligochastes occurred with an average number 

 of 76 per m. 3 Over 50 per cent, of the non-occurrences of oligo- 

 chastes 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 oligqchastes in the 

 plankton. This is in sharp contrast with the nematodes, which 

 appear with rising floods and access of tributary waters. The 

 oligochastes are thus largely adventitious, at times when run-off 

 from vegetation-rich backwaters prevails, and when Lemnacece and 

 Ceratophyllum are washed into the channel by hydrographic changes. 



ROTIFERA. 

 (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. 3 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 



