38 ANAEROBIC ffABITATS 



fusion be avoided. A few examples will illustrate this. 

 Of two species of animals living in the same oxygen- 

 poor habitat, such as the mud of a pond, one, which is 

 equipped with a sufficiently powerful respiratory pig- 

 ment, may lead an aerobic life since, with the help of 

 the pigment, it can secure sufficient oxygen even at low 

 tensions, while the other, lacking a respiratory pig- 

 ment, may be forced to a metabolism characterized by 

 the preponderance of anaerobic reactions. 



Size, too, may determine whether aerobiosis or anaero- 

 biosis prevails. Von Brand (1938) has pointed out that 

 the long controversy as to whether intestinal worms do 

 or do not live anaerobically is rather pointless. Large 

 helminths, like Ascaris, living at the low oxygen tensions 

 prevailing in the intestine, cannot get enough oxygen 

 for a predominantly aerobic life. Small ones, on the 

 other hand, like Ostertagia (Davey, 1938) probably 

 can. Similar considerations apply to tissue parasites. 

 Endamoeha histolytica, after having penetrated the tis- 

 sues, can, without question, secure the same amount 

 of oxygen as any of the surrounding cells of the host. 

 On the other hand, a relatively large arthropod, like the 

 maggot of Cordylohia, may get only a very limited oxy- 

 gen supply as long as it is tunnelling under the intact 

 skin of the host. These examples may suffice; they 

 could easily be multiplied. 



Conversely, one is not justified in assuming that an 

 organism living normally in surroundings rich in oxy- 

 gen will always lead a purely aerobic life. Trypano- 

 somes furnish a case in point. At one period of their 

 life cycle they live in the blood stream of vertebrates, 

 a habitat that certainly is rich enough in oxygen. Nev- 

 ertheless their metabolism is by no means purely aerobic; 

 on the contrary, surprisingly large amounts of sugar 

 are metabolized by them by fermentative processes. 

 Other examples are the malaria parasites where the sit- 

 uation appears to be similar. 



