DIFFERENCES IN ANAEROB. FLNCrrONS 265 



Invertebrates wliicli can live anaerobically either have a 

 mechanism for the excretion of toxic end products or th(»y 

 elaborate non-toxic end products. 



The importance of the excretion mechanism, as an 

 adaptation to anaerobic life, seems to have been first 

 emphasized by Lesser (1912). The facts proving that 

 the ability to excrete organic acids and other end products 

 of the anaerobic metabolism into the medium is widely 

 distributed amongst invertebrates have already been men- 

 tioned previously. The accumulation of acids within cer- 

 tain body fluids should perhaps also be regarded as true 

 excretion, as when, for example, considerable amounts of 

 lactic acid and perhaps other acids accumulate in the 

 fluid of the mantle cavity of clams. 



The interesting question why some invertebrates are 

 able to eliminate these substances from their tissues while 

 others are not can unfortunately not be answered yet. 

 Xo correlation with the morphological structures which 

 serve excr(»tory purposes seems possible. 



In this connection a word should be said also of the 

 elimination of non-toxic end products of anaerobic me- 

 tabolism. Among them we may consider primarily higher 

 fatty acids and true fat. As explained previously such 

 substances represent the main end products in some free- 

 living protozoa, some parasitic worms and arthropods. If 

 the periods of anaerobiosis are not excessively long, the 

 fatty substances may simply be deposited wdthin the body 

 where they r-ause no damage. This happens, for example, 

 in protozoa like Stenfor, or in parasites like the Gasfrro- 

 phihis larva. However, even a completely non-toxic sub- 

 stance does not accumulate indefinitely. One finds, there- 

 fore, that in animals which live permanently in oxygen- 

 free or oxygen-poor habitats and which elaborate fat, the 

 latter is excreted just like a toxic substance would be. 

 This is ilhistrated in the case of Fasciola hepntica. A 

 modified form of this behavior is encountered in JMouiezia 



