80 



somewhat erratic. It has occurred in every month from February 

 through October, but in smaller numbers and sporadically in the 

 colder months. In 1898 its seasonal curve is of characteristic form. 

 It makes its appearance March 1 5 , and is continuously present until 

 the end of September. There is a vernal maximum April 26 of 

 240,000, but no corresponding autumnal one. In 1898 there are 

 indications of recurrent pulses at brief intervals which coincide in 

 location immediately or approximately with similar ones of Gonium 

 and Pandorina. These pulses occur March IS (3,600), April 5 

 (2,800), April 26 (240,000), June 14 (60,000), August 2 (8,000), 

 August 23 (3,200), and September 20 (2,000). The minima 

 between these pulses in all cases but one fall below 1,000. In 

 1897 a vernal pulse was not detected, a maximum of 496,000 

 occurring August 31, and but three mindr pulses appearing. In 

 1896 this species appeared in the plankton on February 20, 

 and remained until the end of August with a month's interrup- 

 tion in May- June. There were no marked pulses, exceeding 

 15,000, in that year. The absence of the spring flood (Pt. I., 

 PI. X.) and the disturbed hydrograph of the summer may account 

 for this suppression of development in Eudorina. The distribu- 

 tion in preceding years is also irregular. 



Eudorina begins its seasonal development at temperatures but 

 slightly above 32, but any considerable growth is not attained 

 until at least 45 has been reached, and the largest pulses on record 

 have been at the close of the period of maximum summer heat at a 

 temperature of 80, and the vernal pulses have been at 60 or above. 

 The disappearance of Eudorina from the plankton in the early fall, 

 about the tim that foliage is killed by autumnal frosts, has been 

 constant in the different years. 



Eudorina is not sufficiently abundant to be of any considerable 

 importance in determining directly the volume of the plankton. 

 It serves as food for many of the rotifers, and is itself frequently 

 parasitized by Dangeardia mammillata Schroder, which destroys the 

 cells but leaves the matrix intact. There are times when it is 

 hardly possible to find perfect colonies, and when it is not unusual 

 to see colonies swimming about propelled by one or two surviving 

 cells. 



Euglena acus Ehrbg.* Average number, 214,807. Found from 

 the middle of March till the first of November, and most abundantly 



