236 Wisconsin Academy of Sciences, Arts, and Letters. 
net whose straining surface was made of No. 20 silk bolting 
cloth. No attempt was made to study those plankton organisms 
which are so small that they readily pass through the meshes 
of the bolting cloth, to which the term nannoplankton has re¬ 
cently been applied. The hauls were quantitative in nature 
and they give a fair idea of the vertical distribution of the vari¬ 
ous forms constituting the net plankton. The number of or¬ 
ganisms in each catch was determined by the counting method. 
This number was multiplied by the factor representing the co¬ 
efficient of the net. This factor was determined by means of a 
tube 3 m. long and 10 cm. in diameter. At the lower end this 
tube was fitted with a sliding door which carried a bolting cloth 
strainer and a removable plankton bucket. The tube was low¬ 
ered into the water while open, then the door was drawn over 
the lower end by means of a line and the tube raised, so that 
the water within it was strained and the plankton concentrated 
in the bucket. Then the net was hauled through the same 
stratum and the number of plankton organisms obtained in the 
two catches was ascertained by counting. 
The results given in table III (p. 246) show the number of 
individuals per cubic meter of water in the various strata. 
Lake Amatitlan.—The phytoplankton of this lake was charac¬ 
terized by the great preponderance of diatoms, of which Melosira 
was by far the most abundant form. It reached a maximum of 
nearly eleven million filaments per cubic meter of water in 
the 0-5 m. stratum. Synedra came next in point of abundance. 
Clathrocystis was the most abundant blue-green alga, but the 
maximum number of this form was found in the bottom stratum. 
This was probably an indication of senility; the form having 
passed its period of maximum development, the senile indi¬ 
viduals had sunk into the lower water. The material from lake 
Amatitlan contained a greater variety of phytoplankton forms 
than that from any of the‘other three lakes. 
Ceratium was the most abundant protozoan. The maximum 
number, more than a million per cubic meter, was found in the 
0-5 m. stratum. 
The rotifers were represented by Triarthra longiseta, Anuraea 
stipitata, and Pedalion fennicum. Triarthra was found only 
in the 15-20 m. stratum, and Anuraea was rather evenly dis¬ 
tributed through all strata of the lake. Pedalion was not found 
in the upper 10 m. but was present at all depths below this 
stratum. It was most abundant in the 20-25 m. stratum. 
