59 
small numbers. It was found six times in March; twice in January 
and April; and once in February, July, and November. Its ap- 
pearance in the plankton is thus predominantly in winter months 
and at temperatures below 45°, though it occurs in the extremes 
of temperature conditions. 
Carchesium lachmannit S. Kent.—Average number, 26,546. This 
is normally an attached species, and its appearance in the plankton 
is due to the detachment of the heads. Small fragments of colonies 
are also found, but the greater number are isolated heads. The 
detachment seems to be a physiological process of the organism and 
not merely the result of accidents. It is thus a detached and an 
adventitious planktont. Many of the heads taken in the plankton 
are in a moribund condition. For example, in a pulse of March, 
1896, the following proportions were recorded. 
Total 
Date Carchesium | Per cent. | Per cent. 
per m.3 normal | moribund 
1896 
Nieto him ly/a ee at ata ter torn aos tensa coe bk cratered 60,420 55 45 
AEE BS cee et hee) See wt me Soe 2 104,535 48 : 5? 
BO) 3. BS SR Bote BA ae a eI terre ALT SS // Al 5 47 
Pec p telah es eksiet Acs ava chased: Phey>,steo) anchecensye byes we 16,688 39 61 
Enumerations were based on the total number of heads, both 
normal and moribund. The colonies are sessile, and adhere in vast 
numbers to any substratum furnishing a suitable place for attach- 
ment—submerged vegetation, brush, sticks, and fishermen’s nets. 
The latter sometimes become so clogged with Carchesium and 
floating mats of Crenothrix and Beggiatoa as to break down in the 
current of the river. How far the number of free heads in the 
plankton is an index of the development of the species in the stream 
can not be determined from the data at hand. 
This species has been taken in the plankton in every month of 
the year, but its occurrences between the early part of May and 
