114 PROTOZOA. 



is plain that the deep sea animals cannot all be living on one 

 another. 



Almost every kind of nutritive relation occurs among the 

 Protozoa. Predatory life is well illustrated by most In- 

 fusorians, and thorough going parasitism by the Gregarines ; 

 Opalina in the rectum of the frog may serve as a type of 

 those which feed on decaying debris, and Volvox of those 

 which are holophytic. Radiolarians, with their partner Algffi, 

 exhibit the mutual benefits of symbiosis, the plants utilising 

 the carbon dioxide of their transparent bearers, the animals 

 being airated by the oxygen which the plants give off in 

 sunlight, and probably nourished by the carbohydrates which 

 they build up. Some of the parasitic forms, especially 

 among the Sporozoa, are of serious importance to higher 

 animals. 



Though Protozoa may be seriously infected by Bacteria, 

 Acineta parasites, some fungi, like Chyiridium, &c., fatal 

 infection is rare, because of the power of intracellular 

 digestion which most Protozoa possess. "The parasite," 

 Metchnikoff says, " makes its onslaught by secreting toxic 

 or solvent substances, and defends itself by paralysing the 

 digestive and expulsive activity of its host ; while the latter 

 exercises a deleterious influence on the aggressor by digesting 

 it and turning it out of the bod)', and defends itself by the 

 secretions with which it surrounds itself" With this struggle 

 should be compared that between phagocytes and Bacteria 

 in most multicellular animals. 



Few Protozoa come into direct touch with human life, 

 but man has several Protozoon parasites, e.g., Atmrba coli, 

 associated with inflammation of the intestinal mucous mem- 

 brane, Coccidium oviforme (Sporozoa), affecting the liver, and 

 various Infusorians. On the other hand, the shells of 

 Protozoa deposited as ooze in ancient days, have formed 

 important deposits, such as chalk and Barbadoes Earth. 



General Zoological Interest. — The Protozoa illustrate, in 

 free and single life, forms and functions like those of the 

 cells which compose the many celled animals. Typically, 

 they show great structural or morphological simplicity, but 

 great physiological complexity. Within its single cell, the 

 Protozoon discharges all the usual functions, while in a higher 

 animal distinct sets of cells have been specialised for various 



