122 



in part at least, of the dates of collection ; but after allowance is made 

 for this, the species still exhibits a seasonal cycle of remarkable regu- 

 larity. The autumnal pulse is of less amplitude, and of less regu- 

 larity in location as to time and temperature. In 1894 it appears 

 September 4 (14,000) at 78; in 1895, on September 12 (5,840) at 

 81; in 1896, on August 29 (58,800) at 74 or October 14 (63,200) 

 at 57; in 1897, on October 5 (204,400) at 71; and in 1898, on 

 September 27 (92,800) at 73. 



The midsummer pulses are, as a rule (Table I.), of less amplitude 

 than the vernal or autumnal ones. In 1896 and 1898 exceptions to 

 this statement appear in two large developments which follow in 

 each case upon the decline of the June rise. In 1896 (Pt. I., PI. X.) 

 this pulse (152,400) came June 11, and in 1898 (Pt. I., PI. XII.) it 

 came (1,499,200) June 7 at 78 and exceeded in amplitude the re- 

 corded vernal pulse. In both cases the pulse was recorded as occur- 

 ring at an interval of a week after the crest of the June rise had 

 passed. The character and sequence of these pulses is well shown in 

 Table I. 



The occurrence of Codonella in abundance in the purer backwaters 

 and in the plankton of our Great Lakes (Kofoid, '95) indicates that it 

 is not dependent upon the sewage bacteria directly for food for its 

 development in our waters. The appearance of the greatest pulses 

 during a period of considerable sewage dilution still further indicates 

 its independence of sewage bacteria. A comparison of the fluctua- 

 tions of the totals of the chlorophyll-bearing organisms with those of 

 Codonella affords some evidence of a correlation between the two. 

 Of 39 pulses which can be traced, in our records in the chlorophyll- 

 bearing organisms, 21 precede and 13 coincide with those of Codo- 

 nella, while in the remaining 5 instances the multiplication of Codo- 

 nella precedes that of the phytoplankton as a whole. Thus in the 

 main the pulses of Codonella follow, or coincide with, those of the 

 phytoplankton. The evidence of this sequence may be followed in 

 Table I. by a comparison of the records of Codonella with those of the 

 total phytoplankton. The sequence indicates that the food of Codo- 

 nella may be found in the phytoplankton, and that these recurrent 

 periods of growth have some connection with the conditions of nu- 

 trition. The seasonal cycle of Codonella is closely followed by the 

 other member of the family found in our plankton Tintinnidium 

 fluviatile. 



