no PROTOZOA THE SIMPLEST ANIMALS. 



Almost every kind of nutritive relation occurs among the 

 Protozoa. Predatory life is well illustrated by most In- 

 fusorians, and thoroughgoing parasitism by the Gregarines ; 

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

 those which feed on decaying de'bris, 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 of serious importance 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. 



Few Protozoa come into direct touch with human life, 

 but man has several Protozoon parasites, e.g. Amxba 

 colt, associated with inflammation of the intestinal mucous 

 membrane ; Coccidium oviforme, a Sporozoon affecting the 

 liver ; and various Infusorians. 



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 

 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. 

 The famous Eozoon canadense of Cambrian rocks is regarded by most as 

 a purely mineral formation. 



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 (Astrorhizidse), that these were 

 succeeded by forms with regularly agglutinated shells, exhibiting types 

 of architecture which were subsequently expressed in lime. 



