24 



The Canadian Field-Naturalist. 



[Vol. XXV. 



grown ripe individuals collected by A. G. 

 Huntsman near Toronto, Ontario, in June 

 1908, probably represent their last ap- 

 pearance in the summer. Young ones (3- 

 10 mm.) were collected on April 10, 1920, 

 near Toronto, Ontario; they therefore 

 probably hatch earlier near Toronto than 

 at Ottawa. Near Ottawa I have not ob- 

 served them later than the month of May, 

 and in all cases they Avere found only in 

 temporary pools or canals caused by snow 

 melting and the overflow of the Ottawa, 

 Gatineau or Rideau Rivers in the spring. 

 As to their occurrence in the United States 

 I refer the reader to Packard's and Ver- 

 rill's articles about them; it is sufficient 

 here to state, that while certain species 

 occur only in the winter and early spring, 

 others are present both in the spring and 

 in the fall, but not in the summer; while 

 again others {Artemia) are found when 

 the water is very warm. 



According to my own field observations 

 during a period of three years along the 

 arctic coast of north-western America I 

 may safely state that there is only one 

 "brood" (generation) per year in the arc- 

 tic; the same is probably true of the sub- 

 arctic zone of this continent; while in 

 southern Canada and the United States 

 two or more broods (generations) per year 

 may occur; though the long time during 

 which the ponds are dried up during the 

 summer here probably restricts the num- 

 ber of generations considerably. 



Apart from the enemies in (insects, 

 fishes) and outside (birds) the water a 

 great number of fairy-shrimps (and Notos- 

 traca as well) are killed off prematurely 

 in the summer or autumn by the drying up 

 of (at least in the arctic) the particular 

 small pond in which they live or by being 

 thrown up along the margin of the part- 

 icular lake by weaves in windy Aveather, as 

 I have repeatedly observed in the arctic 

 parts of America and Greenland. I have 

 also observed how a great number of phyl- 

 lopods in the fall freeze into the ice as the 

 latter begins to form and grows in thick- 

 ness, though a number of individuals were 

 living in the water right under the ice, 

 even if there were only a few inches of free 

 w'ater. It will, therefore, be realized how 

 important it is for the propagation of these 

 animals, that they occur in such vast num- 



bers and that the hatching of the eggs takes 

 place almost immediately after the melt- 

 ing of the pond or lake-ice in the spring, 

 or after the autumn rains (absent in the 

 arctic) have filled the dried up reservoirs 

 in which the eggs are lying. 



The fairy shrimps on this continent are 

 divided into two groups (super-families-) 

 according to the number of their pre- 

 genital, foliaceous body-legs. The one 

 group {Poly art emiidcB) has 17 to 19 pairs 

 of these while the other group (comprising 

 the great majority of fairy-shrimps spe- 

 cies) has only 11 pairs. 



To the first group belong two genera, of 

 which one {Poly arte mi a, 19 pairs of foli- 

 aceous legs) is not found in America, but 

 a species (P. forcipata Fisch.), occurs in 

 the arctic parts of Europe and Asia, both 

 in Scandinavia and Siberia and probably 

 also in the intervening arctic part of Rus- 

 sia ^ Its biology, structure and develop- 

 ment have been given in detail by G. O. 

 Sars, in Fauna Norivegiae, 1896, pp. 59-65. 



The genus found in America is Pohjar- 

 temieUa, so-called OAving to its similarity to 

 Polyartcmia, from which it, however, is 

 distinguished by having tAvo pairs less of 

 foliaceous legs. Curiously enough the 

 genus Poly art emiella seems to be limited 

 to the arctic and subarctic parts of Alaska 

 and Yukon Territory, and thus resembles 

 someAvhat the fresh Avater Amphipod -S^?m- 

 rella. In the same Avay as S. ^ohanseni has 

 its nearest relatives in Europe and -Asia, 

 so have also the tAvo knoAvn species of 

 Polyartemiella their nearest relative in the 

 Eurasian form Polyartemia mentioned 

 above. Considering their respective dis- 

 tribution Ave may perhaps assume, that 

 both Symirella Ri\d Polyartemia have their 

 original home in Eurasia, and have spread 

 from there to the northAvest corner of 

 America, AA'here then the latter genus be- 

 came transformed in the course of time to 

 the only slightly different genus Polyarte- 

 miella. This invasion of America took 

 place perhaps via a former land-connection 

 betAveen Siberia and Alaska, a view Avhich 

 is supported by the fact, that one of the 

 Polyartemiella species (P. judayi) has been 

 found upon at least some of the islands in 

 the Bering Sea (De Dees). 



3. Recorded from Novaja Semlja (Hansen). 



