100 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
A striking characteristic of Lake Erie, as exhibited during the summer of 1898, 
was the poverty in limnetic Rotatoria. In others of the Great Lakes the Eotifera have 
been found to form at times a considerable portion of the plankton. In Lake Erie the 
proportion formed by them was so small as to be hardly noticeable. The following 
12 limnetic forms were seen; noue of them were abundant and most of them were 
rare: Floscularia mutabilis Bolton, F.pelagica Rousselet, Conocliilus unicornis Rousse- 
let, Asplanchna priodonta Gosse, Synchceta stylata Wierzejski, Polyartlira platyptera 
Ehr., Notops pelagians n. sp., Anurcea cochlearis Gosse, Notliolca longispina Kellicott, 
Plcesoma lenticulare Herrick, Gastropus stylifer Iinhof, and Anapus ovalis Bergen dal. 
This list contains one species not hitherto found in others of the Great Lakes, namely, 
Notops pelagicus n. sp. On the other hand, the following species found in others of the 
Great Lakes were not found in Lake Erie: Conocliilus volvox Ehr Asplanchna her- 
riclcii de Guerne, Plcesoma hudsoni Imhof, Notommata monopus Jennings, and Anurcea 
aculeata Ehr. The first of these has been found in Lake Erie by another observer; 
the others have not. 
The littoral, bottom, and swamp Rotifera of Lake Erie are, on the other hand, 
very abundant, as shown by the fact that 120 species have already been recorded 
from this lake. The shallow and marshy parts of the lake teem with rotifers. 
Investigation of the Rotifera is as yet far too incomplete to draw very positive 
conclusions in regard to the geographical distribution of these animals. The evidence 
of all kinds, so far as it goes, seems to indicate that the following is true: Whether any 
given rotifer shall be found in a given body of water depends, not upon the locality 
of this body of water, not upon its connection with or separation from any particular 
drainage system, but upon the conditions present in that particular body of water. In 
stagnant swamps all over the world are likely to be found the characteristic Rotifera 
of stagnant water, with little regard to the country in which the swamp is found; 
in clear lake water will be found everywhere the characteristic limuetic Rotifera; in 
sphagnum swamps everywhere the sphagnum Rotifera. Variation in the rotifer 
fauna of different countries is probably due to variation in the conditions of existence 
in the waters of these countries, not to any difficulty in passing from one region to 
another. The Rotatoria are thus potentially cosmopolitan, any given species occur- 
ring wherever ou the earth the conditions necessary to its existence occur. The num- 
ber of different sorts of Rotifera to be found in any given region depends, then, upon 
the variety of conditions to be found in the waters of this region. Two bodies of 
water a half mile apart, presenting entirely different conditions, are likely to vary 
more in the make-up of their rotifer faunas than two bodies of water 5,000 miles apart 
that present similar conditions. Of course on this view it is likely that the Tropics 
will have many characteristic species not found in the cooler regions, since the Tropics 
present many conditions of existence not found elsewhere, and the same may be true 
of Arctic regions. The problem of the distribution of the Rotifera is, then, a problem 
of the conditions of existence, not a problem of the means of distribution. The 
ability of the eggs to live in dried mud, which may be carried about on the feet of 
birds or blown about as dust by the winds, seems to give sufficient opportunity to 
any species to multiply wherever occur the conditions necessary to its existence. 
The fact that different rotifers do thus require different conditions for existence 
is, of course, evident to all who have worked on the group. A striking example of 
the fact that it is the life conditions, not the means of distribution of the animals, that 
determines the character of the fauna in a given body of water is given by a com- 
