1920] AIle7i: Plankton of the San Joaquin River 83 



Stentor coeruleus Ehrbg. 



station I Station II Station III Daily Hourly 



Average 8,141 ' 123 141 492 



Identification usually satisfactory. Under conditions of counting 

 probably some confusion with 8. nigcr and perhaps another species. 

 This was because distortion in preservation made it almost impossible 

 to use any distinction except size in many cases, hence a small S. 

 coenilcus might be mistaken for S. niger, or a large specimen of the 

 latter might be mistaken for the former. Generally, however, the 

 stouter body of S. coeruleus showed plainly enough to make it fairly 

 certain. Occurrence was almost confined to first four months and 

 December. Only two catches were recorded at Station II and three 

 at Station III. Clearly a cold water form almost limited to sewage 

 or at least to water with heavy organic content. Maximum occurred 

 in December at Station I, although the catches were fairly constant 

 over a period of four months from the first of the year. The maxi- 

 mum for this early period was in January. Since it is absent at 

 periods of greatest stagnation, this planktont is evidently more influ- 

 enced by temperature than by that factor. The most favorable tem- 

 perature seems to be at about 10° C. since the maxima just mentioned 

 come at about that condition. 



Stentor niger Ehrbg. 



station I station II Station III Daily Hourly 



Average 42,691 739 871 472 



Identification satisfactory. Probably some loss through net. Also 

 considerable loss of this and the preceding form through clinging to 

 net, utensils, etc. This form was first recorded as S. roeselii on account 

 of the nucleus, but the delicacy and grace of form together with 

 smoky color seem so strongly characteristic that it has finally been 

 referred to S. niger. The decision to make the change was mainly due 

 to Professor Kofoid's statement that nuclear characters are very 

 unstable in this genus. They cannot be determined at least while 

 counting. 



Occurrence at Station I runs later in spring and begins earlier in 

 fall than that of S. coeruleus. The numbers are very noticeably 

 greater in the fresh material while the animals are active. The Jan- 

 uary and December maxima fall on the same dates as those of S. 

 coeruleus but there are strong pulses in April and May by way of 

 contrast. It seems safe to say then that both species have about the 

 same optimum but that S. niger is able to endure a slightly higher 

 temperature (20° C.) and that it is less disturbed by flood conditions. 



