Muttkowski—The Fauna of Lake Mendota. 
391 
A distinction must be made for the small Oligochaeta which 
are given in bulk. They form a very conspicuous percentage 
of the numerical total of the August fauna, altho less promi¬ 
nent as regards their bulk. As a rule they live in loosely con¬ 
structed cases of accretions and secretions on the aquatic 
plants, and were it not for the difficulty of gathering them 
and freeing them from their cases, an individual count might 
be attempted. They are not difficult to recognize, for each of 
the several species of Chaetogaster, Nais, Pristina, etc., which 
make up their membership, has a distinctive locomotion, usu¬ 
ally of the S-type, which makes identification of living speci¬ 
mens comparatively simple. ' Pish eat a large number of them, 
but it is difficult to estimate the percentage, for only the mi¬ 
nute setae remain undigested. 
Of the bottom Oligochaeta, Limnodrilus claparedeianus 
Ratzel, Tubifex tubifex Muller, Lumbriculus limosa, etc., are 
eaten with the mud by perch, suckers, and other fish. Spar- 
ganophilus eiseni Smith, the blue earthworm, is confined to 
gravelly areas and has been taken up to depths of 2.5 m. 
7. Hirudinea. (Table 5, p. 474.) 
The distribution of leeches in the lake was found to be more 
uniform and less restricted than at first suspected, except for 
Erpobdella punctata (Leidy) and Nephelopsis obscura Verrill. 
Altho the percentage of specimens is much larger in rocky areas 
a great many individuals are fomid among the plants of the 
littoral depths. 
Erpobdella and Nephelopsis, curious to say, find their opti¬ 
mal conditions on the shore margin, where coarse gravel and 
stones are intermixed. Stones buried at the point where wa¬ 
ter and shore meet seem their favorite habitat choice; for un¬ 
der these rocks, of about a size of 25 by 15 cm., one may occa¬ 
sionally find clusters of as many as 85 individuals on a single 
rock. The stones are often tightly wedged and the leeches 
evince surprising agility and strength in burrowing under 
them. The egg-cases of Erpobdella and other species are 
common on the stones of rock and gravel shores. 
The food of leeches is variable, and may consist of the mi¬ 
crofauna and flora of the lake as well as of some of the larger 
lake forms. Thus I have found may-fly larvae (Hexagenia, 
