121 



in PL XLV. of Part I.) might for this reason tend to interfere with 

 the development of Carchesium, and thus cut off the source from 

 which the plankton individuals arise. I am not able, however, 

 to trace any close correlation between the fluctuations of the chem- 

 ical matters indicative of sewage and sewage decay and those of 

 Carchesium. In the stable hydrographic conditions of 1897 we find 

 a symmetrical pulse of considerable dimensions rising from 2,200 

 on November 9 to 283,800 on December 7, and declining to 26,500 

 on January 11, 1898. Stable low water with an ice blockade 

 (Pt. I., PI. XI. and XII.) characterize this season. ~Nr> explanation 

 for the fluctuation is suggested in the physical environment. The 

 chemical condition of the water, was, however, greatly disturbed 

 (Pt. I., PL XLIV.). The fivefold increase in free ammonia is indic- 

 ative of approaching stagnation under the ice, and the threefold 

 increase in chlorine marks the sewage concentration. Approaching 

 stagnation might have caused the decline of Carchesium, or it may be 

 a specific reproductive cycle of the organism which combines with 

 the external factors of the environment to produce such a wave of 

 occurrence. 



Chilodon cucullulus Ehrbg. Average number, 102. This species 

 was found in the plankton in January and February during the bac- 

 terial increase. It was also found in July. It escapes through the 

 silk net, and does not ordinarily appear in plankton collections, 

 though abundant wherever decay is active. 



Codonella cratera (Leidy). Average number, 101,024 or 452,500*. 

 This is the most abundant of the ciliates in our plankton, consti- 

 tuting about one third of their total number. It appears in 

 every month of the year, and in 1898 it was recorded in every 

 collection but one, that of December 13 (Table I.). It is sub- 

 ject to great fluctuations in numbers, its maximum occurrences tend- 

 ing to appear in April, May, or June, and again in September or 

 October. Minimum numbers prevail during the winter, when many 

 of the shells are empty, and the midsummer interval is subject to 

 pulses of varying amplitude. Spring pulses were detected as follows : 

 in 1895, on April 29 (16,324) at 64; in 1896, on April 24 (562,152) at 

 72;inl897,onApril27(470,000)at60;andinl898,onMay3(736,000) 

 at 60. These vernal pulses coincide with or approximate closely to 

 the dates of the spring volumetric pulses. This somewhat remark- 

 able approximation of dates near the end of April may be the result, 



