97 



bution (see Table I.) that Amoeba may become an active member of 

 the plankton during the warmer seasons, like other Rhizopoda, as a 

 result, perhaps, of the formation of gas or oil vacuoles in its proto-' 

 plasm. Of the 30 occurrences, 21 fall between ApriLlS and Octo- 

 ber 17, with water temperatures of 58 and 56, respectively. Of 

 these 21 occurrences in warm waters but 8 accompany flood inva- 

 sions, while all of the 9 occurrences during the colder months are in 

 connection with such disturbances. Finally, the maximum num- 

 ber per cubic meter (6,400) was found July 21 in clear waters, free 

 from the debris of flood invasion. In conclusion, it seems probable 

 that Amoeba in warmer seasons of the year (above 56) may adopt 

 a limnetic habit. There is, however, the possibility that local and 

 minor disturbances of the water due to current, waves, etc., are 

 the occasion of its presence in the plankton in the absence of flood 

 conditions. Jennings ('OOa) reports both A. proteus and A. radiosa 

 in the open water of Lake Erie. 



The range of temperature of river water in which Amoeba was 

 found was from 32 to 89 the full extremes observed by us in 

 the river at Havana. The temperature at the maximum occur- 

 rence, July 21, 1897, was 82. It is perhaps significant that 14 

 of the 30 occurrences of Amoeba were between June 21 and Sep- 

 tember 6, the period of maximum heat, the river averaging 

 almost 80 apparently the optimum temperature for the occur- 

 rence of Amoeba in the plankton in this locality. The relative 

 numbers of individuals found in the various collections of the five 

 years are too irregular to suggest any conclusions as to a seasonal 

 cycle. 



Amoeba verrucosa Ehrbg. Average number, 19. This species 

 was found but three times in the plankton, once each in May, 

 August, and September, occurring but singly, and in each case in 

 flood waters. It is apparently a tycholimnetic member of the plank- 

 ton. The temperature limits of its recorded occurrence in the 

 plankton were 5.8 and 82 respectively. 



Arcella. 



This genus is represented in the plankton by four species and 

 two varieties which, like most of the Rhizopoda, are exceedingly 

 variable, grading in some instances into each other by occasional 



