The Flagellates 107 



always draw the body after them, these colonies swim 

 along with open ends forward, apparently in defiance 

 of all the laws of hydromechanics, rotating slowly on 

 the longitudinal axis of the colony as they go. Dino- 

 bryon is of an amber yellow tint,. and often occurs in 

 such numbers as to lend the same tint to the water it 

 inhabits. It attains its maximum development at 

 low temperatures. In the cooler waters of our larger 

 lakes it is present in some numbers throughout the year, 

 though more abundant in winter. Kofoid reports it as 

 being "sharply limited to the period from November to 

 June" in Illinois River w T aters. Its sudden increase 

 there at times in the winter is well illustrated by the 

 pulse of 1899, when the numbers of individuals per 

 cubic meter of water in the Illinois River were on suc- 

 cessive dates as follows: 



followed by a decline, with rising of the river. 



Dinobryon often develops abundantly under the ice. 

 Its optimum temperature appears to be near o° C. It 

 thus takes the place in the economy of the waters that 

 is filled during the summer by the smaller green 

 flagellates. 



Symira (fig. 30^) is another winter flagellate, similar 

 in color and associated with Dinobryon, much larger 

 in size. Its cells are grouped in spherical colonies 

 united at the center of the sphere, and equipped on the 

 outer ends of each with a pair of flagella, which keep the 

 sphere in rolling locomotion. The colonies appear at 

 times of maximum development to be easily disrupted, 

 and single cells and small clusters of cells are often found 

 along with well formed colonies. Synura when abund- 

 ant often gives to reservoir waters an odor of cucumbers. 



