115 



was not unusual to find as high as ten or fifteen per cent, of the 

 individuals parasitized, and a number of empty loricas bearing addi- 

 tional testimony to their destructive agency. 



Bertram ('92) describes these structures as " parasitische 

 Schlauche" in the body cavity of rotifers, and Przesmycki ('01) 

 works out their life history, and describes the organisms as Dimoe- 

 rium hyalinum, but does not designate their systematic position or 

 affinities. There are, however, marked suggestions of sporozoan 

 affinities in the organism found in the rotifers of the Illinois plankton, 

 which seems to be identical with that described by Przesmycki ('01). 



Obviously it is difficult to take a census of such internal para- 

 sites. A record was kept, however, of the number of parasitized 

 individuals in each species of rotifer, and references will be made to 

 these results in the discussion of the hosts. Dimcerium appeared 

 in both summer and winter rotifers, and its seasonal distribution 

 naturally depends upon the number of available hosts. It was in 

 consequence most abundant during the midsummer and autumn 

 months. 



CILIATA. 



Average number, 15,812,346, including filter-paper collections. 

 If these be excluded and the silk catches only averaged, the number 

 will fall to less than a tenth of this sum. The ciliates are found in 

 the plankton of the Illinois throughout the whole year, and as' a 

 whole they do not exhibit any common seasonal predominance. The 

 analysis of the distribution of the individual species which follows, 

 exhibits two diverse tendencies which affect the distribution of the 

 totals. These are the vernal and autumnal pulses of the Tintinnidce, 

 represented by Codonella cratera and Tintinnidium fluviatile, and 

 the autumnal-winter occurrence of a large number of species during 

 the height of the sewage contamination and bacterial development. 

 The dominant species in this ciliate wave are Carchesium lachmanni, 

 Epistylis, Amphileptus, Lionotus, Plagiopyla nasuta, Glaucoma 

 scintillans, Stentor niger, and 5. c&ruleus. Some species, as Halteria 

 grandinetta, have a wider seasonal distribution, and others, as 

 Vorticella, Trichodina, Zoothamnium, Pyxicola affinis, and many 

 others, are adventitious in the plankton. Still others, as Rhabdo- 

 styla, Cothurniopsis vaga, Operculana, and similar peritrichan 

 parasites, are passive members of the plankton. The actively 



(9) 



