40 SFRYEY OF IWTJRTIJBRATE^ 



Spathidium lieherkuhni, Perispira ovum, Lacrymaria 

 aquae-dulcis and Dactylochlamys pisciformis. 



In the upper layers of the shallow sapropelic waters, 

 varying amounts of oxygen do occur and many other pro- 

 tozoa are found there, as shown in the Appendix, Table 

 1, but it seems likely that these will have to endure from 

 time to time periods of severe or even complete lack of 

 oxygen. The same is probably true of the green forms 

 which, during the night, cannot obtain oxygen through 

 photosj-nthesis. 



It is unfortunate that the culturing of typical sapro- 

 pelic ciliates has so far not been possible; a real knowl- 

 edge of the oxygen relationships of these animals can be 

 expected only from experiments under controlled lab- 

 oratory conditions. We have, at the present, only some 

 indications, from studies by Wetzel (1928) and by Lieb- 

 mann (1936a) that members of the genera Metopus, 

 Caenomorpha, Plagiopyla and Discomorpha are injured 

 by oxygen. 



A protozoan fauna similar in some respects to the 

 sapropelic fauna is found in sewage tanks (Lackey, 

 1924, 1925, 1926, 1932 ; Liebmann, 1936, 1936a). But again 

 it should be emphasized that not every protozoon found 

 there is by necessity adapted to a permanently anaerobic 

 life. Some animals are carried into the tanks with the 

 incoming, more dilute and oxygen-containing sewage ma- 

 terial. Such organisms will die rather rapidly or form 

 cysts as soon as the oxygen disappears completely. Others 

 are truly anaerobic; they occur regularly in the deeper 

 anaerobic parts of the tanks and thrive there {of. Ap- 

 pendix, Table 2). 



Lackey (1932) made an experimental comparative 

 study of the behavior of an Opercularia species, which 

 normally is abundant in the aerobic trickling filters 

 but is not found, in the. motile form, within the tanks, 

 and of the flagellate Trepomonas agilis, which is char- 

 acteristic of the deeper anaerobic layers of the lat- 



