SURVEY OF INVERTEBRATES 101 



composition of the water, these observations are difficult 

 to evaluate in regard to the anaerobic survival during the 

 experimental periods. 



Comparable cyst formations have been observed also 

 in other copepods, especially in Harpacticoids (Lauter- 

 born and Wolf, 1909 ; Kessler, 1912 ; Donner, 1928 ; Moore, 

 1939; Deevey, 1941). The last-named investigator found 

 no hatching from cysts of CantJiocamptus staphylinoides 

 during experimental anaerobic periods lasting 5 to 15 

 days at temperatures between 10 and 20.5° C. He is of 

 the opinion 'Hhat encystment is a constant feature of the 

 life cycle of CantJiocamptus, but that the time of onset 

 and of emergence is under environmental control, so that 

 encystment is an efficient form of anaerobiosis. " 



Planktonic marine crustaceans may prove to have well- 

 developed anaerobic functions; a better knowledge of the 

 fauna in the layer of minimum oxygen concentration in 

 the Pacific might reveal interesting adaptations. Pseu- 

 docalanus elongatus and Calanus finmarchicus, as Nik- 

 itin (1931) has observed, penetrate into layers of the 

 Black Sea where the oxygen concentration is well be- 

 low 1 cc. per liter. Furthermore, Marshall, Nicholls and 

 Orr (1935) reported that the oxygen consumption of Ca- 

 lanus finmarchicus begins to decline when the concentra- 

 tion of oxygen sinks below 3 cc. per liter. The conclu- 

 sion that, in the Black Sea, this animal should be forced 

 to supplement the failing aerobic metabolism by anaero- 

 bic processes seems almost inescapable. 



Another interesting, though as yet less analyzed case 

 is that of Mysis relicta. This animal occurs both in fresh 

 and in sea water. The minimum oxygen concentration 

 for its well-being is 4 cc. per liter in fresh water, while 

 in the Baltic Sea the oxygen concentration of the deep 

 layers in which the animal is mostly found is only 

 around 1.6 cc. per liter (Thienemann, 1928). These ob- 

 servations give rise to the following questions: (a) Is 



