​Zoology is the branch of biology that studies all aspects of the animal kingdom​:

• Structure - How the animal is shaped, its molecular structure
• Embryology - development of embryos and fetuses (babies)
• Evolution - changes in heritable traits of biological populations over successive generations
• Classification - the arrangement of animals and plants in taxonomic groups according to their observed similarities
• Habits - behavior and usual ways of living and migration patterns for a particular species
• Distribution - where the animals live
• Ecosystems - dependencies, how a community of living things function together

The largest animal is the blue whale (Balaenoptera musculus). This ocean-dwelling creature can weigh up to 210 tons or about 420,000 pounds! (In metrics this amounts to 190 metric tonnes or 172,365.101 kilograms.)

The largest land animal is the African bush elephant (Loxodonta africana).The smallest know animals can only be seen through a microscope. Among the smallest species is a fish parasite, the Myxobolus species of microrganism.

​The smallest is the Myxobolus shekel which only grows to be about 8.5 microns. To view this nearly invisible creature you need to get your hands on an electron microscope. The electron microscope uses a wavelength of accelerated electrons to illuminate up to 100,000 times visible light.