Invertebrates are animals that do not have a backbone or internal skeletons, such as spiders, insects, lobsters, and crabs. Estimates of 97% of all animal species are invertebrates. They contain the greatest number of animal species. So far, almost 2 million species have been described, most of which are insects. Amazingly, there are millions more to be discovered. Invertebrates are multicellular organisms. Most invertebrates are aquatic (found in water).
In fact, it is believed that invertebrate originated from water. Most invertebrates change form as they grow, they go through a process known as metamorphosis. All invertebrates are cold-blooded animals. This means they cannot keep body temperature constant all the time. Most invertebrates have an open circulatory system where blood flows in an open cavity. They also possess a simple respiratory system, which consists mostly of trachea or gills.