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