46 GEOL. AND NAT. HIST. SURVEY OF MINNESOTA. 



Lake Superior Entomostraea, 1891 j Marsh: On tJie Deep- Water Crustacea 

 of Green Lake, 1891; Forbes: A Preliminary Report on the Aquatic In- 

 vertebrate Fauna of the Yellowstone National Park, 1893; Marsh: On the 

 Cyclopidce and Calanidce of Central Wisconsin, 1893. (?) 



"While it is probable that comparatively little remains to be done 

 in the systematic study of I^orth American Copepoda except in the 

 west and in mountainous and brackish stations, yet there are few 

 more attractive fields than that offered by the study of the biological 

 conditions under which these forms appear. The processional ap- 

 pearance of several species in the same pool in the spring, the effects 

 of stagnation on the organization, and the peculiar heterogenesis or 

 polymorphism due to age, are all capable of yielding valuable addi- 

 tions to theoretical biology. While much care and attention are 

 requisite, and considerable collateral information, yet the Gladoceran 

 studies of Professor Weismann afford a suitable model and induce- 

 ment. In a purely systematic paper like the present one no attempt 

 can be made to indicate these lines of work. 



FAMILIES OF THE FRESH-WATER COPEPODA. 



I. Calanidae. Body elongate; abdomen more slender and quite 

 distinct from the thorax. Antennae long, 23 to 25-jointed; in the 

 male that of the right side modified for prehension. First four pairs 

 of feet two-branched, outer branch three-jointed. Fifth feet several- 

 jointed, those of the male prehensile; dissimilar on the two sides. 

 Ovisac single. Habit strongly natatory. 



II. Cyclopidse. Cephalothorax compact, ovoid; abdomen slender. 

 Antennse usually shorter than the cephalothorax, both geniculate in 

 the male, 8- to 18-jointed. Second antennae (antennules) one-branched. 

 Fifth feet rudimentary, a sixth pair sometimes present, especially in 

 the male. Ovisacs double, lateral. Habit natatory. 



III. Harpacticidae. Body cylindrical or flattened; abdomen not 

 sharply distinct. Antennae short, not more than 10-jointed, those of 

 the male both geniculate. Antennules palpate. First pair of swim- 

 ming feet usually prehensile. Fifth feet foliaceous, somewhat dissimi- 

 lar between the sexes. Ovisac usually single. Habit creeping or 

 sub-natatory. 



FAMILY CALANIDJ:. 



This group is pre-eminently marine and contains diverse and 

 graceful forms, mostly with very elongated bodies and antennae. Of 

 the six genera here enumerated as more or less habituated to the use 

 of fresh water, two are found as yet only in America and one is con- 

 fined to Europe. 



