The Lake as a Microcosm. 88 
The crustacean fauna of these lakes is more varied than that of any other 
group. About forty species were noted in all. Crawfishes were not especially 
abundant, and all captured belonged to a single species —Cambarus virilis. 
Two amphipods occurred frequently in our collections; one, less common here 
but very abundant farther south—Crangonyxz gracilis—and one, Allorchestes 
dentata, probably the commonest animal in these waters, crawling everywhere 
in myriads over the submerged water plants. An accasional Gammarus fascia- 
tus was also taken in the dredge. A few isopod Crustacea occur, belonging to 
Mancasellus tenax, Harger,—a species not previously found in the state. 
I have reserved for the last the Entomostraca,— minute crustaceans of a sur- 
rising number and variety, and of a beauty often truly exquisite. They be- 
ong wholly, in our waters, to the three orders, Copepoda, Cladocera, and Os- 
tracoda,— the first predaceous upon still smaller organisms and upon each 
other, and the two others chiefly vegetarian. Twenty-one species of Clado- 
cera have been recognized in our collections, these representing sixteen genera. 
It is an interesting fact that twelve of these species are found also in the fresh 
waters of Europe. Five cyprids have been recognized, two of them common 
to Europe, and also an abundant Diaptomus, a variety of a European species. 
Several Cyclops were collected which have not yet been determined. 
These Entomostraca swarm in microscopic myriads among the weeds along 
the shore, some swimming freely, and others creeping in the mud or climbing 
over the leaves of plants. Some prefer the open water, in which they throng 
locally like flocks of birds, coming to the surface preferably by night, or on 
dark days, and sinking to the bottom usually to avoid the sunshine. These 
pelagic forms, as they are called, are often exquisitely transparent, and hence 
almost invisible in their native element,—a charming device of Nature to 
protect them against their enemies in the open lake, where there is no chance 
of shelter or escape. Then with an ingenuity in which one may almost detect 
the flavor of a sarcastic humor, Nature has turned upon these favored children 
and endowed their most deadly enemies with a like transparency, so that 
wherever the towing net brings to light a host of these crystalline Cladocera, 
there it discovers, also, swimming, invisible, among them, a lovely pair of 
robbers and beasts of prey — the delicate Leptodora and the OCorethra larva. 
These slight, transparent, pelagic forms are much more numerous in Lake 
Michigan than in any of the smaller lakes, and peculiar forms occur there com- 
monly, which are rare in the larger lakes of Illinois and entirely wanting in 
the smallest. 
The vertical range of the animals of Geneva Lake showed clearly that the 
barrenness of the interiors of these small bodies of water was not due.to the 
greater depth,—or at least not to that alone. 
While there were a few species of crustaceans and caseworms which occurred 
there abundantly near shore, but rarely, or not at all, at depths greater than 
four fathoms, and may hence be called littoral species, there was, on the whole, 
little diminution either in quantity or variety of animal life, until about fif- 
teen fathoms had been reached. Dredgings at four and five fathoms were 
nearly or quite as fruitful as any made. On the other hand, the barrenness of 
the bottom at twenty to twenty-three fathoms was very remarkable. The 
total products of four hauls of the dredge and one of the trawl at that depth, 
aggregating fully a mile and a half of continuous dragging, would easily go 
into a two-dram vial, and represented only nine species of animals— not 
counting dead shells and fragments which had probably floated in from shal- 
lower waters. The greater part of this little collection was composed of speci- 
mens of Lumbriculus and larve of Chironomus. There were a few Corethra 
larvee, a single Gammarus, three small leeches, and some sixteen mollusks, all 
but four of which belonged to Pisidium. The others were two Spheriums, a 
Valvata 3—- carinata, and a V. sincera. None of the species taken here were 
