SUM3IARY 135 



3. The initial stimulation of the eggs preparatory to 

 cleavage does not require molecular oxygen (in Arhacia), 

 but the cleavage process itself does. 



Molluscs. 



1. Many lamellibranchs are found in surroundings very 

 poor in oxygen and are able to withstand for long periods 

 an experimental deprivation of oxygen, 



2. Lamellibranchs of the tidal zone lead a predomi- 

 nantly anaerobic life during low tide, but gastropods oc- 

 curring in the same habitats do not. 



3. It seems quite certain that lamellibranchs kept in 

 anaerobic conditions have no special source of oxygen of 

 their own. 



4. A certain ability to live auaerobically is developed 

 both in terrestrial and in aquatic gastropods, but there 

 is no direct parallelism between that ability and the oc- 

 currence of anoxic conditions in the normal habitat of 

 these animals. 



5. The anaerobic functions of cephalopods are very 

 poorly developed; these organisms never penetrate an- 

 aerobic habitats. 



Arthropods. 



1. Copepods and, to a lesser degree, cladocerans are 

 found quite often in oxygen-free surroundings. Some 

 species also survive the experimental deprivation of 

 oxygen. 



2. Other resistant forms are found among ostracods 

 and cirripedians, while decapods are in general more 

 sensitive. 



3. Some aquatic insect larvae {Don acta, chironomids, 

 Chaohorus and others) have extremely well developed 

 anaerobic functions. 



4. The question of the factors enabling some chirono- 

 mids to establish themselves in habitats very poor in 



