134 



The average number in channel waters is 103 per m. 3 , and, as 

 might be expected, their occurrences are erratic in seasonal distri- 

 bution and their numbers are irregular. They occurred in channel 

 waters in eveiy month of the year and throughout the whole seasonal 

 range in temperatures. The numbers in 1898 were larger and occur- 

 rences more frequent in May, during the run-off of the spring flood, 

 and smaller and more erratic during the rest of the year. In the 

 total of all collections enumerated the percentage of occurrences was 

 highest in June (60 per cent.), July (83 per cent.), August (48 per 

 cent.), and October (47 per cent.), and lowest in colder months, when 

 it rarely rises above 30 per cent. The numbers are also larger in the 

 warmer months, a maximum record of 19,250 per m. 3 on September 

 4, 1894, following a slight rise in river levels at low stages. The 

 adventitious character of the Turbellaria in channel plankton is sug- 

 gested by the erratic data, but the adaptability, at least of certain 

 species, to the limnetic habit under certain conditions is also indi- 

 cated by the large numbers. 



The identification of the Turbellaria in plankton collections is not 

 feasible in the course of the usual methods of examination of pre- 

 served plankton. Accordingly no effort was made to identify the 

 individuals occurring in our catches. Many of them were evidently 

 rhabdoccele turbellarians, and of these probably many were Stenos- 

 toma leucops. The genus Vortex was also represented. 



Mesostomum ehrenbergii O. Schmidt was taken in small numbers 

 on August 26, 1895, along the shores of the river in vegetation. This 

 identification is that of Dr. W. McM. Woodworth ('97). 



Stenostoma leucops O. Schmidt. Average number, 21. By far the 

 greater proportion of the turbellarians in our collections probably 

 belong to this species. The statements made regarding the group as 

 a whole therefore probably apply to this species. 



TREMATODA. 



Many of our predaceous fishes and other aquatic vertebrates are 

 infested to an extraordinary degree by flukes parasitic in the intestine 

 or other viscera. This, in conjunction with the fact that the fish 

 markets are located in house-boats along the stream and their refuse 

 generally cast directly into the channel, is sufficient to account for the 

 few adventitious adult distomes which have been noted in our plank- 



