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 lachmanni, engulfing the 

 head in 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- 

 chesium 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 



