1920] Allen: Plankton of the San Joaquin River 89 



At Station II this forin was recorded in every month except Feb- 

 ruary, April, May and June though the numbers were rather small 

 throughout and the catches mostly at irregular intervals. The maxi- 

 mum occurred in October at a temperature of 20° C. and again at 

 17° C. Conditions were much the same at Station III except that 

 April, June and July were the only months without catches. This 

 again supports the conclusion that flood and higher temperatures are 

 both deterrent factors. Tlie maximum came in October. 



While the habits of this organism would lead one to think of it as 

 really adventitious in the plankton, the large numbers here raise a 

 definite question as to the validity of that vieW. Can it be that this 

 animal has adopted a free swimming habit under our local conditions? 

 Or are the general conditions so favorable for development that favor- 

 ite haunts become rapidly overcrowded thus forcing individuals tem- 

 porarily into the plankton while seeking other quarters ? 



Rotaria rotatoria Pallas. 



station I St:ition II Station III Daily Hourly 



Average 287,186 19,099 18,283 54,197 8,500 



Identification uncertain. Probably includes two or three other 

 species and possibly other genera. So far as estimated from living 

 material the above named species was more numerous ; the others could 

 not be distiuguislied from it while counting. Occurrence in every 

 month of the year at all stations, though rather light in April, May 

 and first half of June at each, and continuing so through June and 

 July at Stations II and III. The maximum came in March at all 

 three stations. The fairly rapid decline after the March maximum 

 was apparently due to the incoming mountain flood waters. Tliere was 

 a marked pulse in July at the disappearance of these flood waters. 

 Higher temperatui'es undoubtedly kept the numbers down, however, 

 and none of the pulses of the last half year reached the numbers com- 

 mon in the first quarter year. This, of course, is also some indica- 

 tion that the comparative stagnation of the last half year was less 

 favorable than the disturbed hydrographic conditions of the first 

 three months when there was enough rainfall to cause considerable 

 local variation. Comparison with the 1914 condition when there was 

 heavier rainfall may help to settle some of these questions. The num- 

 bers clearly show that this form does best in sewage. There can be 

 no question that it prefers temperatures below 20° C. and the indica- 

 tions are that it does best in waters slightly disturbed, as by local 

 rains, but this cannot be ^settled now. 



No other forms of Bdelloida were recorded. 



