Many different species of animals differing in many respects inhabit the earth and many species have lived and become extinct during past geological time. The first clear cut enumeration of all animals was made by Linnaeus in 1758 who included 4,236 different kinds of animals. At present about 2 million species of animals have been named and about 10,000 new species are added to the list practically every year.
The great animals belonging to the animal kingdom differ in their size, external appearance (morphology), internal structure (anatomy), habits, habitats, physiological specialisations, modes of reproduction and geographical distribution.
The size of animals vary from the smallest, which may be about 3 to 6 microns (1/25,000 of an inch), to the largest, which may be a blue whale reaching a length of nearly 100 feet. The variation in morphological appearance of animals from amoeba to man is remarkable; so also are the differences in their internal structure. Animals are varied in their nutritional habits: some are herbivorous whereas some are pure carnivores; some are parasitic whereas others may be free living. Some animals are gill breathing and others are lung breathing, some are active during the night (nocturnal) while many are active during the day (diurnal). Animals are found in almost all types of habitats: they are aquatic, amphibious, terrestrial and many show adaptations for flight. They are found in the coldest regions of the world and also in the hot deserts.