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 1897, under the most pro- 
nounced type of such conditions. 
Dinobryon is a common planktont in the Great Lakes (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 
ite teauced 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. Inthe pulse of February 21, 1899, the average num- 
ber of cells in the colony falls from thirteen to sixteen, during the 
foewOr une 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. Dzinobryon is some- 
times covered with large numbers of minute choanoflagellates, 
probably Salpingeca 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 
