545 



merged water-plants. An occasional Gammariis fasciatus was also 

 taken in the dredge. A few isopod Crustacea occur, belonging to Manca- 

 sellus tcnax — a species not previously found in the state. 



I have reserved for the last the Entomostraca — minute crustaceans 

 of a surprising number and variety, and of a beauty often truly exquisite. 

 They belong wholly, in our waters, to the three orders, Copedoda, Ostrac- 

 oda, and Cladocera — the first two predaceous upon still smaller or- 

 ganisms and upon each other, and the last chiefly vegetarian. Twenty- 

 one species of Cladocera have been recognized in our collections, repre- 

 senting sixteen genera. It is an interesting fact that twelve of these spe- 

 cies are found also in the fresh waters of Europe. Five cyprids have been 

 detected, two of them common to Europe, and also an abundant Diapto- 

 mus, a variety of a European species. Several Cyclops species 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 shoals of fishes, coming to the 

 surface preferably by night, or on dark days, and sinking to the bottom 

 usually by day 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 shel- 

 ter or escape. Then with an ingenuity in which one may almost detect 

 the flavor of 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 Corethra 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 commonly which are rare in the larger lakes of Illinois 

 and entirely wanting in the smallest. The transparent species are also 

 much more abundant in the isolated smaller lakes than in those more 

 directly connected with the rivers. 



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 alone. While there were a few species 

 of crustaceans and case-worms 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 dimi- 

 nution either in quantity or variety of animal life until about fifteen fath- 

 oms had been reached. Dredging, at four or five fathoms were nearly 

 or quite as fruitful as any made. On the other hand, the barrenness 

 of the i)(>tt(im at twenty to twonty-llircc fatlionis was very remark- 

 able. The total product of four hauls of the dredge and one of the 



