iia 
in Pl. XLV. of Part I.) might for this reason tend to interfere with 
the development of Carchestum, 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., Pl. XI. and XII.) characterize this season. No explanation 
for the fluctuation is suggested 1n 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 1t 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 (Tablé 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° ;in 1897,on April 27 (470,000) at 60°; and in 1898, on May 3(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, 
