77 



of our summer waters are inimical to Dinobryon. That its absence 

 from the plankton at that time is not due merely to low- water con- 

 ditions is shown by the December pulse in 189 7, under the most pro- 

 nounced type of such conditions. 



Dinobryon is a common planktont in the Great-Xakes (Kofoid, 

 '95) during the summer months, but surface temperatures here 

 rarely exceed 68, and are 10 to 20 below those of the Illinois 

 River. In German lakes Apstein ('96) finds the maximum develop- 

 ment of Dinobryon in June and a continuance through the summer 

 in reduced numbers, but temperatures are also 10 to 20 (F.) 

 lower than in our waters. In the case of D. stipitatum there is a 

 second maximum in August. Lauterborn ('93) finds Dinobryon 

 throughout the winter in the plankton of the Rhine, with a maxi- 

 mum in April-May, with diminished numbers during the summer, 

 and a second maximum in September. 



The filter-paper collections give very much larger numbers, 

 owing partly to the inclusion of small colonies which escape 

 through the meshes of the silk net in the usual method of collec- 

 tion. The numbers are increased at least thirty-fold if filter col- 

 lections are utilized instead of silk, as above. 



The size of the colonies in the collections varies greatly, the 

 averages ranging from three to forty-eight cells. The maximum 

 pulse is attended or followed by a considerable decrease in the size 

 of the colony. In the pulse of February 21, 1899, the average num- 

 ber of cells in the colony falls from thirteen to sixteen, during the 

 rise of the pulse, to seven, at its culmination. On the pulse of 

 May 10, 1898, the average is thirteen, and a week later, when the 

 pulse declines from 16,153,600 to 43,200, the average size of the 

 colony drops to three cells. Cysts also are most frequent during 

 and subsequent to maximum development. Dinobryon is some- 

 times covered with large numbers of minute choanoflagellates, 

 probably Salpingosca minuta Kent. Frequently colonies occur in 

 which only the younger cells are alive. 



Dinobryon is, in the light of its distribution, one of the impor- 

 tant synthetic planktonts of the colder months, and is one of the 

 primal links in the chain of food relations of that season, serving 

 as food for some of the winter Cladocera and Copepoda. The fact 

 that its maxima frequently occur when volumetric minima appear 

 as, for example, on February 21, 1899 indicates that Dinobryon 



