118 
DISCUSSION OF SPECIES OF CILIATA. 
Amphileptus spp.—Average number, 630. Amphileptus is a well- 
defined winter planktont in the river at Havana, and it affords a 
striking instance of the interdependency of organisms in the plank- 
ton. It feeds upon the heads of Carchesium lachmannt, engulfing the 
head im situ and encysting during digestion. Such heads, joined to 
the colony or free in the plankton, have been found in our waters. Its 
seasonal distribution at Havana is almost identical (Table I.) with 
that of Carchesium, upon which it feeds. Thus in 1897-98 Car- 
chestum was continuously present in the plankton from October 26 
to May 10, with a pulse on December 7 of 283,800, and one on 
February 8 of 197,600. Amphileptus appears October 26; continues, 
with interruptions, to. May 17; and has pulses December 7 and 
January 25, the latter reaching 13,545. In 1898-99 both appear 
early in October and have coincident pulses on November 22 and 
January 24. In 1895-96 the interdependence is even more striking, 
Carchesium reaching a greater development in this winter, with a 
pulse of 964,600 on November 27, and Amphileptus reaching 14,469 
on this date and 14,835 a week later. Both species decline during 
the flood which follows, and rise during March to culminations, on 
the 24th, of 104,535 and 3,636, respectively. 
In 1898, Amphileptus disappears on April 12 at 52°, save for an 
isolated occurrence May 17 at 64°. It does not reappear until 
October 18 at 52°. In 1897, it reappeared October 26 at 59°, and 
in 1895-96 its limits were 45° and 48°, with the exception: of one 
occurrence, April 17, at 66°. Carchesium occurs irregularly and 
sparingly during summer months, and Amphileptus was not taken 
in the plankton during that period. Its occurrence in the plank- 
ton is limited in the main to temperatures below 50°, but this 
limitation may be due primarily to the reduced numbers, at higher 
temperatures, of the organism upon which it feeds. It appears 
during the period of greatest sewage-contamination and bacterial 
development in the river at Havana. Roux (’01) finds Amphilep- 
tus most abundant in stagnant waters about Geneva in the winter 
months. 
Aspidisca costata (Duj.) Stein.—Found in the plankton but once 
—Jan. 11, 1898, at 32°. 
Bursaria truncatella O. F. Mull.—Average number, 23. This 
large ciliate was found in the plankton at irregular intervals and in 
