102 SURVEY OF IX} ERTEBRATES 



the oxygen needed by the tissues supplied so much more 

 easily in salt water than in fresh water {cf. Thienemann, 

 l. c.)1 (b) Is the oxygen requirement greater in fresh 

 water than in salt water {cf. Schlieper, 1929) I (c) Does 

 a partial transition to anaerobiosis take place in the sea? 

 Only experimentation will bring an answer. 



Small crustaceans that burrow in the bottom material 

 must also sometimes find themselves in very difficult re- 

 spiratory conditions, Moore (1931), who studied the dis- 

 tribution of Harpacticoids, especially Danielssenia typica 

 and Cletodus longicaudatus, and of ostracods in the mud 

 of the Clyde Sea in areas where the interstitial water was 

 free from oxygen, found these two copepods and several 

 ostracods in the surface layer mostly; very few were 

 found at depths below 1 cm. It is doubtful whether the 

 uppermost layers were completely free of oxygen and 

 since the copepods, according to Moore, showed only lit- 

 tle resistance in experimental anaerobic conditions, one 

 cannot assume that they normally live in the complete 

 absence of oxygen. But there is no reason to doubt that 

 they must be adapted to life at low oxygen tensions. 



Fresh-water ostracods seem to be even more resistant. 

 Juday (1908) found living Candona in lakes during the 

 stagnation periods; Moore (1939) reported Candona 

 exilis, Candona reflexa, Cypria exsculpta and Cypria la- 

 custris from a similar habitat and Ward (1940) found 

 Cypria elegantula in the oxygen-free water of a pond. 

 Experimentally, ostracods proved to be very resistant, as 

 Lindeman (1942) has shown. 



Little is known about isopods. Aselhis aquaticus has 

 been found in oxygen-free water during the winter (Al- 

 sterberg, 1930) ; and the occurrence of Glyptonotus ento- 

 mon in layers of the Baltic Sea poor in oxygen has been 

 assumed to be linked with well-developed anaerobic func- 

 tions (von Buddenbrock, 1939). But the last-named or- 



