The Lake as a Microcosm. 85 
markable amphipod crustaceans also belong in a peculiar way to this deep 
water. In the European lakes the same Mysis occurs in the deepest part, with 
several other forms not represented in our collections,— two of these being 
blind crustaceans related to those which in this conntry occur in caves and 
wells. 
Comparing the other features of our lake fauna with that of Europe, we 
find a surprising number of Entomostraca identical; but this is a general phe- 
nomenon, as many of the more abundant Cladocera and Copepoda of our 
small wayside pools are either European species, or differ from them so slight- 
ly that it is doubtful if they ought to be called distinct. 
It would be quite impossible within reasonable limits to go into details 
respecting the organic relations of the animals of these waters, and I will con- 
tent myself with two or three illustrations. As one example of the varied 
and far-reaching relations into which the animals of a lake are brought in the 
general struggle for life, I take the common black bass. In the dietary of this 
fish I find, at different ages of the individual, fishes of great variety, represent- 
ing all the important orders of that class; insects in considerable number, 
especially the various water bugs and larve of day-flies; crawfishes, fresh- 
water shrimps, and a great multitude of Entomostraca, of many species and 
genera. The fish is therefore directly dependent upon all these classes for its 
existence. Next looking to the food of the species which the bass has eaten, 
and upon which it is therefore indirectly dependent, I find that one kind of 
the fishes taken feeds upon mud, Algze, and Entomostraca, and another upon 
nearly every animal substance in the water, including mollusks and decompos-~ 
ing organic matter. The insects taken by the bass, themselves take other 
insects and small Crustacea. The crawfishes are nearly omnivorous, and the 
other crustaceans, some of them eat Entomostraca and some Alge and Proto- 
zoa. At only the second step therefore, we find our bass brought intodepend- 
ence upon nearly every class of animals in the water. 
And now, if we search for its competitors we shall find these also extremely 
numerous. In the first place, I have found that all young fishes of every 
description feeds at first almost wholly on Entomostraca, so that the little bass 
finds himself at the very beginning of his life engaged in a scramble for food 
with all the other little fishes in the lake. In fact, not only all young fishes, but 
a multitude of other animals as well, especially insects and the larger Crustacea, 
feed upon these Entomostraca, so that the competitors of the bass are not con- 
fined to members of its own class. Even mollusks, while they do not directly 
compete with it, do so indirectly, for they appropriate myriads of the micro- 
scopic forms upon which the Entomostraca largely depend for food. But the 
enemies of the bass do not all attack it by appropriating its food supplies, for 
many devour the little fish itself. A great variety of predaceous fishes, tur- 
tles, water-snakes, wading and diving birds, and even bugs of gigantic dimen- 
sions destroy it on the slightest opportunity. It is, in fact, hardly too much 
to say that fishes which reach full maturity are relatively as rare as centen- 
arians among human kind. 
As an illustration of the remote and unsuspected rivalries which reveal 
themselves on a careful study of such a situation, we may take the relations 
of fishes to the bladder-wort,—a flowering plant which fills many acres of the 
water in the shallow lakes of northern Illinois. Upon the leaves of this 
species, are found little bladders—several hundred to each plant— which 
when closely examined, are seen to be tiny traps for the capture of Entomos- 
traca and other minute animals. The plant usually has no roots, but lives 
entirely upon the animal food obtained through these little bladders. Ten of 
these sacs which I took at random from a mature plant contained no less than 
ninety-three animals (more than nine to a bladder), belonging to twenty-eight 
different species. Seventy-six of these were Entomostraca, and eight others 
