100 SURVEY OF INVERTEBRATES 



cuspidatus was reported from a similar locality by Imel 

 (1915). 



Several Cyclops species proved to be quite resistant 

 also in experimentally induced anaerobic conditions (Ta- 

 ble 13), as shown by the figures of Huss (1913), Nik- 

 itinsky and Mudrezowa-Wyss (1930) and Lindeman 

 (1942). It appears very likely, therefore, that some, but 

 certainly not all, species of Cyclops, and perhaps other 

 copepods, as well as some cladocerans, are able to live 

 for relatively long periods in the absence of oxygen in 

 nature. Before coming to a definite conclusion, however, 

 it is desirable to study more thoroughly the oxygen re- 

 lationships of the organisms involved. Several factors 

 that may prove serious objections to the view of a marked 

 ability of such animals to live anaerobically will there- 

 fore be considered in the following paragraphs. 



One is whether the animals really stay for any length 

 of time in the truly anaerobic layers. They are good 

 swimmers and might from time to time come to the sur- 

 face strata which are never entirely devoid of molecular 

 oxygen. 



Another objection that might render doubtful the sig- 

 nificance of the long survivals reported by Huss (1913) 

 and Lindeman (1942) is that it is not sufficiently clear 

 whether the animals found motile at the end of the 

 experiments did not pass at least part of the time in 

 resting stages. This possibility should be considered 

 seriously; it was given some attention by Huss but the 

 problem was not adequately solved. Birge and Juday 

 (1908) and Moore (1939) observed that in nature Cy- 

 clops hicuspidatus regularly forms a cocoon in which 

 immature specimens lead a latent life for several months. 



It is important to note that these animals were able to 

 hatch even when the lake water was still oxygen-free. 

 But since neither the hatching nor the formation of the 

 cocoons can be correlated with any definite change in the 



