546 



trawl at that depth, aggregating fully a mile and a half of continuous 

 dragging, would easily go into a two-dram vial, and represents only nine 

 animal species — not counting dead shells, and fragments which had 

 probably floated in from shallower waters. The greater part of this little 

 collection was composed of specimens of Lumbriculus and larvze of Chi- 

 ronomus. There were a few Corethra larvje, a single Gammarus, three 

 small leeches, and some sixteen mollusks, all but four of which belonged 

 to Pisidium. The others were two Sphseriums, a Valvata carinata, and a 

 V. sincera. None of the species taken here are peculiar, but all were of 

 the kinds found in the smaller lakes, and all occurred also in shallower 

 water. It is evident that these interior regions of the lakes must be as 

 destitute of; fishes as they are of plants and lower animals. 



While none of the deep-water animals of the Great Lakes were 

 found in Geneva Lcike, other evidences of zoological affinity were de- 

 tected. The towing net yielded almost precisely the assemblage of species 

 of Entomostraca found in Lake Michigan, including many specimens 

 of Limnocalanus macrunis Sars; and peculiar long, smooth leeches, 

 common in Lake Michigan but not occurring in the small Illinois lakes, 

 were also found in Geneva. Many FaJvata tri-carinata lacked the middle 

 carina, as in Long Lake and other isolated lakes of this region. 



Comparing the Daphnias of Lake Michigan with those of Geneva 

 Lake, Wis. (nine miles long and twenty-three fathoms in depth), those 

 of Long Lake, 111. (one and a half miles long and six fathoms deep), 

 and those of other, still smaller, lakes of that region, and the swamps 

 and smaller ponds as well, we shall be struck by the inferior development 

 of the Entomostraca of the larger bodies of water in numbers, in size 

 and robustness, and in reproductive power. Their smaller numbers and 

 size are doubtless due to the relative scarcity of food. The system, of 

 aquatic animal life rests essentially upon the vegetable world, although 

 perhaps less strictly than does the terrestrial system, and in a large and 

 deep lake vegetation is much less abundant than in a narrower and shal- 

 lower one, not only relatively to the amount of water but also to the 

 area of the bottom. From this deficiency of plant life results a defi- 

 ciency of food for Entomostraca, whether of algje, of Protozoa, or of 

 higher forms, and hence, of course, a smaller number of the Entomos- 

 traca themselves, and these with more slender bodies, suitable for more 

 rapid locomotion and wider range. 



The difiference of reproductive energy, as shown by the much 

 smaller egg-masses borne by the species of the larger lakes, depends 

 upon the vastly greater destruction to which the paludal Crustacea are 

 subjected. Many of the latter occupy waters liable to be exhausted by 

 drought, with a consequent enormous waste of entomostracan life. The 

 opportunity for reproduction is here greatly limited — in some situations 

 to early spring alone — and the chances for destruction of the summer 

 eggs in the dry and often dusty soil are so numerous that only the 

 most prolific species can maintain themselves. 



