SURVEY OF INVERTEBRATES 59 



That protozoa parasitizing tissues proper should, as 

 a rule be able to obtain at least some oxygen from 

 their surroundings appears probable but whether it will 

 suffice to allow a purely aerobic metabolism is at the 

 present stage of our knowledge a matter of speculation. 



The only data available concerning the oxygen rela- 

 tionships of tissue parasites were obtained with Coccidia. 

 They indicate very definitely that oxygen is available to 

 the tissue-stages of the j^arasites. The immature oocysts 

 of many species eventually reach the intestinal lumen, 

 the lumen of enlarged bile ducts, or the gall bladder of the 

 host — places where they can get only insignificant 

 amounts of oxygen. It is characteristic that the oocysts 

 of these species do not mature inside the host but only in 

 the outside w^orld w^here an adequate oxygen supply be- 

 comes available (Balbiani, 1884; Pfeiffer, 1892; Metzner, 

 1903). On the other hand, these oocysts are quite re- 

 sistant under experimentally induced anaerobic conditions 

 (Metzner, 1903). In other species the oocysts remain in 

 the tissues of the host, those of Eimeria subepithelialis, 

 for example, in the subepithelial tissues. Moroff and 

 Fiebiger (1905), while studying this species, observed 

 that its oocysts mature in situ, and they assumed that 

 maturation would not require oxygen. Later on, how- 

 ever, Fiebiger (1913) reached the tentative conclusion 

 that the oxygen present in the host tissues might be suf- 

 ficient to allow the maturation process to take place. The 

 idea that the maturation or the non-maturation of the 

 coccidian oocysts inside the host depends solely on the 

 different amounts of oxygen available to the different spe- 

 cies has been expressed by Reichenow (1929). 



