If only what you know about invertebrates is that they are animals without a backbone, then you should know that there are other characteristics of invertebrates that you can use to differentiate them from vertebrates. First, Invertebrates do not have to live internal skeletons to give shape to their bodies. Although some of them have a soft body, others like arthropods have in their outside body a rigid structure that gives support to their bodies. Arthropods and some other invertebrates use exoskeletons, while others use hydrostatic skeletons to give support to their bodies. Second, invertebrates constitute about 96 percent of living animal species on earth.
While close to 2 million invertebrates have been identified so far, researches have shown that there are still millions of invertebrates that are yet to be identified. Some invertebrates don't have a regular shape; a perfect example is an amoeba; they have the ability to change their body shape in any direction. Invertebrates do not have cell walls. Some invertebrates are immobile, while others can move from one point to another.
All invertebrates have four standout traits. They do not possess cell walls, like other animals, they produce offspring by two reproductive cells, or gametes coming together to create a new organism of their species, they do not have a spinal column, and they are multi-cellular. About 80 percent of the invertebrates can be found in terrestrial habitats.
Some are unusually small, and the body covering of invertebrates is simple. Invertebrates are usually creatures of various shapes. For example, the amoeba has an abnormal body shape, which changes continuously. Sponges exhibit a plant-like appearance. The invertebrate creatures demonstrate a wide range of variation in size. Another characteristic of invertebrates is the presence or absence of germ layers.
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.