Arthropoda
The arthropods are by far the most successful animal phylum, both in variety of habitats and in numbers of species and individuals, including such familiar creatures as crabs, spiders and insects in general. They've successfully adapted to life in water, on land and in the air. About 80% of all known animal species are arthropods - about 800,000 species of them have been described and recent estimates put the total number of species in the phylum as roughly 6 million. Arthropods are found in a greater variety of habitats than any other animal group; on top of mountains, at great depths in the ocean, in the icy wilderness of Antarctica and climbing up the overflow pipe of your bath. They can survive great extremes of temperature, toxicity, acidity and salinity.
They have a segmented body covered by a jointed external skeleton (exoskeleton), with paired jointed appendages on each segment. Because the jointed exoskeleton can't grow, it has be shed regularly as the arthropod grows. This phenomenon, called moulting, or ecdysis, is very useful to the creature; it can grow rapidly and change its form until the new skin hardens. Though of course it is also much more vulnerable to predators until it has its armour back. Arthropods can go through many different forms in their life cycles, such as caterpillar to butterfly.