It is quite hard to believe that almost 97% of all living animal species are creatures without backbones called invertebrates. Surprisingly invertebrates were the first species to evolve. Butterflies, star fish, crabs, earth worms, jelly fish, lady bugs, spiders, mosquitoes, corals etc are all invertebrates. In vertebrates, skeleton made of bones acts as a protection for the internal organs. Most invertebrates lack skeletons while some have skeletons that are not made of bones but with some other soft substances. Their sizes vary from microscopic to more than 50 feet.
There are two groups of invertebrates that are known for their external skeletons. External in the sense, they don’t stiffen the muscles or the internal organs but instead they just enclose whole body of the organism. The first group includes snails and mollusks that have a shell made of calcium carbonate. The other group includes insects, spiders and crab with external skeleton made of a substance called chitin which is chemically similar to plant cellulose. Some are parasitic in nature like ticks and some are internal parasites like tapeworms.
The biggest of invertebrates is the giant squid that live in the sea. Lacking backbone, the invertebrates gain structural support for their bodies in many ways. Some invertebrates maintain their body shape by a tough, flexible cuticle and high internal pressure. They are cold blooded meaning their body temperature depends on the surrounding temperature. After reproduction, they change form as they grow going through a process known as metamorphosis. It is a process by which an animal develops physically (change their form) after birth and as they mature. Both adults and young ones have different way of living including how and what feed on.