A SUPERB example illustrating the ecological niche is afforded by the distribution of the plankton copepod Limnocalanus macrurus, a freshwater relict derived from the marine arctic species L. Grimaldii. The animal has an upper temperature limit of 14° C, and a lower limit of dissolved oxygen of 4 c.c./l. oxygen. In the lowland lakes beyond the arctic region it will therefore during summer stagnation live between the upper waters which are too warm and the lower waters which are too depleted of oxygen.