196 THE PROTOZOA 



or by the splitting up of complex chemical substances ; the result 

 in either case is the production of energy in various forms and of 

 simple chemical substances, such as water and carbon dioxide 

 (compare Barratt, 148). For the processes of oxidation the 

 organism may either absorb free molecular oxygen from its environ- 

 ment, or may produce it by internal molecular changes of substances 

 contained in its own body, as in anaerobic organisms living in a 

 medium in which free oxygen is lacking. 



Many free-living Protozoa require oxygen, and are visibly and 

 rapidly affected by the lack of it, especially in their powers of 

 movement. No special organs of respiration are found in any 

 Protozoa, being unnecessary hi animals of such small bulk, and in 

 which, consequently, the surface of the body is considerable in 

 proportion to the mass. The contractile vacuoles, when present, 

 are doubtless a means of eliminating carbon dioxide, together with 

 other waste products, from the body. It must be supposed, there- 

 fore, that as a general rule oxygen is taken up from the surrounding 

 water by the protoplasm, of which the limiting membranes are 

 freely permeable, and that the carbon dioxide is given off in a 

 similar manner. The experiments of Verworn (211) on /Spirostomum 

 show that the respiratory processes take place in the cytoplasm, 

 independently of the nucleus, which takes no share in respiration. 



On the other hand, many sapropelic (p. 14) and parasitic forms 

 inhabit media lacking in free oxygen, and are anaerobic ; in such 

 forms the respiratory processes of the protoplasm can only take place 

 by intramolecular changes, in which the stored-up reserve-materials 

 are probably split up to supply the required oxygen. 



The experiments of Putter (201) on a number of species of Ciliata, both 

 free-living and parasitic, showed that, when these animals were placed in 

 an anaerobic environment, different individuals of the same species reacted 

 very differently to the conditions, some dying very rapidly, others being 

 quite unaffected for a long time. It was shown further that this difference 

 was related to the amount of reserve -materials present in the body (proteins 

 and glycogen), which can be observed to vary greatly in different individuals 

 from the same culture. If Paramecia were first starved for some days and 

 then placed in anaerobic conditions, they succumbed much more rapidly 

 than normal individuals. Moreover, under anaerobic conditions the reserve- 

 materials were used up much* more rapidly than under normal conditions, 

 and without resulting in increased production of energy. Opalina, when 

 placed in a culture-medium to which albumen was added by boiling up dried 

 white of egg in salt-solution, was able to make use of the energy of the albumen 

 without the help of free oxygen, and so to live for a much longer time. The 

 ciliates were found to succumb much more rapidly to the effects of anaerobic 

 conditions in smaller than in larger quantities of water, as the result of auto- 

 intoxication in consequence of the defective excretion of the products of 

 anaerobic metabolism. Spirostomum was found to be more affected by 

 anaerobic conditions in small quantities of water than Paramecium. The 

 differences between the two forms is to bo ascribed to the system of the 

 contractile vacuoles, which is far more efficient in Paramecium than in 

 Spirostomum ; the contractile vacuoles tend to remove from the body the 



