92 SURVEY OF INVERTEBRATES 



Clams living in the tidal zone usually close their 

 shells tightly when the ground is uncovered. Mitchell 

 (1912) showed that the oxygen consumption of oysters 

 that have shut their valves is not greater than that of 

 empty shells, that is, than the oxygen consumption of 

 microorganisms growing on the shells. This means that 

 the oxygen uptake of the living tissues has ceased. In 

 other species, Mya arenaria and Venus mercenaria, the 

 oxygen consumption stopped only if the valves were 

 closed artificially. Venus especially continued to con- 

 sume oxygen when the valves were closed by the ani- 

 mal itself. Nevertheless, even in this case, the oxygen 

 intake seems to be insufficient to support a purely aerobic 

 metabolism. The recent work of Dugal (1939), which 

 led to the conclusion that there is an anaerobic metabol- 

 ism in this species, was performed with Vemis speci- 

 mens kept in air. This, of course, caused them to keep 

 their shells closed as tightly as possible. 



Altogether considered, it seems very likely that during 

 low tide many lamellibranchs have a more or less anaero- 

 bic metabolism. This is an interesting difference be- 

 tween lamellibranchs and the jDreviouslj^ discussed worms 

 occurring in the same habitat. 



B. Type of life of lamellihranchs in the absence of 

 oxygen. Whether clams found in media where there is 

 no molecular oxygen, or such as are brought experimen- 

 tally under anoxic conditions, actually lead an anaerobic 

 existence, in the true sense of the word, has been ques- 

 tioned. It has sometimes been assumed that they have 

 some special sources of oxygen. 



Cole (1921) found that mud containing decaying plant 

 tissues was able to oxidize guaiacum in the darkness and 

 in the absence of molecular oxygen. He is of the opinion 

 that the oxidizing agent may be continually formed in the 

 bottom layers. There were some indications that this oxi- 

 dant might be atomic oxygen which then would be avail- 



