119 



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 lachmanni 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. 



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 



