The Infusoria have at the present day an almost universal distribution, being found in all collections of fresh and salt water, where decaying organic matter is present, and where the other conditions of life are favourable. A few are parasites in the interior of other animals (Opalina), but the true affinities of these are doubtful. Owing to the fact of their generally wanting any hard structures which could have been preserved in a fossil condition, no true Infusoria* can be said with certainty to have existed in former periods of the earth's history, though they have doubtless abounded in past time as now. The only possible exceptions to this statement are certain microscopic bodies which occur in the Chalk-flints, and which Ehrenberg considered to be the protective carapaces of Peridinium and allied forms of flagellate Infusoria.