REPRODUCTION OF PROTOZOA. TI9 
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 ; 
Opatina in the rectum of the frog may serve as a type of 
those which feed on decaying débris, and Volvox of those 
which are holophytic. Radiolarians, with their partner 
Algee, exhibit the mutual benefits of symbiosis, the plants 
utilising the carbon dioxide of their transparent bearers, the 
animals being aérated 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 Acinefa parasites, by some fungi, like Chytridiwm, 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 ; 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. 
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 (Astrorhizidze), that these were 
succeeded by forms with regularly agglutinated shells, exhibiting types 
of architecture which were subsequently expressed in lime. 
Relics of siliceous Radiolarian shells are also known from Silurian 
strata onwards, with, perhaps, the exception of the Devonian. Best 
known are those which form the later Tertiary deposits of Barbados 
earth, from which Ehrenberg described no fewer than two hundred and 
seventy-eight species. ~ 
