ii6 PHYLUM PROTOZOA THE SIMPLEST ANIMALS. 



natural death. Conjugation is the necessary condition of their eternal 

 youth. 



It must be noted, however, that some subsequent investigators have 

 watched over two hundred asexual generations of ciliated Infusorians 

 without seeing the slightest trace of senile degeneration. 



Bionomics. Many Protozoa raise organic debris once 

 more into the circle of life, and many form part of the food 

 of higher animals. Thus those pelagic Foraminifera and 

 Radiolarians, which dying sink to the great oceanic depths, 

 form along with more substantial debris the fundamental 

 food supply in that plantless world. Fundamental, since it 

 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 thoroughgoing parasitism by the Sporozoa ; 

 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 

 Algae, exhibit the mutual benefits of symbiosis, the plants 

 utilising the carbon dioxide of their transparent bearers, the 

 animals being aerated 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 fatally injurious to higher 

 animals. 



Though Protozoa may be seriously infected by Bacteria, 

 by Acineta parasites, by some fungi, like Chytridium, etc., 

 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 digest- 

 ing it and turning it out of the body, and defends itself by 

 the secretions with which it surrounds itself." With this 

 struggle should be compared that between phagocytes and 

 Bacteria in most multicelluiar animals. 



History. Of animals so small and delicate as Protozoa, we do not 

 expect to find distinct relics in the much-battered ancient rocks. But 

 there are hints of Foraminifer shells even in the Cambrian ; more than 



