
tek 
was not unusual to find as high as ten or fifteen per cent. of the 
individuals parasitized, and a number of empty loricee bearing addi- 
tional testimony to their destructive agency. 
Bertram (792) 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 Dime- 
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. Dimerium 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. 
CRETATEAS 
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 Tintinnide, 
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 lachmannt, 
Epistylis, Amplileptus, Lionotus, Plagiopyla. nasuta, Glaucoma 
scintillans, Stentor niger, and S. ceruleus. Some species, as Halteria 
grandinella, have a wider seasonal distribution, and others, as 
Vorticella, Trichodina, Zoéthamnium, Pyxtcola affinis, and many 
others, are adventitious in the plankton. Still others, as Rhabdo- 
styla, Cothurniopsis vaga, Opercularia, and similar peritrichan 
parasites, are passive members of the plankton. The actively 
(9) 
