Spiders are invertebrates. The main distinguishing feature of all invertebrates is a lack of spine, or any bones at all. Instead of bones, spiders and similar creatures have exoskeletons made up of plates that cover their bodies and support their internal organs.
Although they look like insects, spiders belong to a group of invertebrates called arachnids. Along with their fellow arachnids, including ticks, scorpions and mites, spiders have eight legs and two main body sections. Their exoskeletons are made up of chitin, a strong, flexible, fibrous substance that is also found in the exoskeletons of insects and crabs and in the cell walls of fungi.