144 PHYLUM PROTOZOA — THE SfMPLEST ANIMALS 



of higher animals. Thus those pelagic Foraminifera and 

 Radiolarians, which sink dying 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 ot 

 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 moreover nourished by the carbohydrates 

 which they build up. Some of the parasitic forms, especi- 

 ally 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 multicellular 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 ; riiore than 

 hints in the Silurian and Devonian ; and an abundant representation 

 in rocks of the Carboniferous and several subsequent epochs. The 

 shells of calcareous Foraminifera form an important part of chalk 

 deposits. 



There seem at least to be sufficient relics to warrant Neumayr's 

 generalisation in regard to Foraminifera, that the earliest had shells 

 of irregularly agglutinated particles (Astrorhizida;), that these were 

 succeeded by forms with regularly agglutinated shells, exhibiting types 

 of architecture which were subsequently expressed in lime. 



