1 882.] and Adjacent Waters. 643 



This is the farthest extreme of a development of the head, 

 which, beginning with such forms as pellncida, runs through <W- 

 cata, apicata, bcroliucnsis, vitrei, kahlcnbcrgcusis and ccderstremii to 

 the present species, where it reaches truly enormous proportions. 

 The meaning of such a character I am not able to imagine. The 

 expansion of the head is a thin and flexible plate, affording lodg- 

 ment to no organs, and seems an utterly useless encumbrance. 



In Geneva lake, Wisconsin, the most abundant entomostracan 

 in October,- was an extremely variable Daphnia approaching 

 hyalina on the one hand and retrocurva on the other, but still sep- 

 arable from both. It is evident that this group of helmeted 

 Daphnias is still in process of active evolution, and it is possible 

 that there are no actual breaks anywhere along the line from 

 hyalina to retrocurva, although in the former the head may be 

 scarcely larger than in D , , / /> /< r, while in lIk Litter it is often 

 more than half as large as the body. 



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, Ills, (one and a half miles long 

 and six fathoms deep), and those of other still smaller lakes of 

 that region, a curious progressive predominance of the large hel- 

 meted forms is very evident in passing from larger to smaller 

 lakes. If we extend the comparison further, and include the 

 other entomostraca, and the swamps and smaller ponds as well, 

 we shall be struck by the inferior development of the entomos- 

 traca of the larger bodies of water, in numbers, in size and robust- 

 ness, and in reproductive power. Their smaller numbers and 

 size are doubtless due to the relative scarcity of food. The sys- 

 tem of aquatic animal life rests essentially upon the vegetable 

 world, although perhaps less strictly than does the terrestrial sys- 

 tem ; and in a large and deep lake vegetation is much less abun- 

 dant than in a narrower and shallower one, not only relatively to 

 the amount of water but also to the area of the bottom. (In all 

 the lakes which I have dredged, life of all sorts was much more 

 scanty in the interior deeper portions than along the margins.) 

 From this deficiency of plant life results a deficiency of food for 

 entomostraca, whether of Algae, of Protozoa or of higher forms, 

 ar id hence, of course, a smaller number of the entomostraca 

 themselves, with more slender bodies suitable for more rapid 

 locomotion. 



