Many species of animals, including humans and other mammals, have an internal support structure made up of bones. This endoskeleton can provide support for a wide range of different body shapes and sizes, and can grow smoothly with the animal it is inside of. Spiders, like all arthropods, lack an internal skeleton and instead make use of an external support.
Instead of having an internal skeleton made of bones, spiders have an exoskeleton. Exoskeletons are hard "shells" made of a substance called chitin that cover the outside of an arthropod and provide structure, protection and support. These exoskeletons cannot grow with the animal, but must instead be shed periodically as the spider gets larger in a process known as molting.
Benefits of Exoskeletons
Exoskeletons have several advantages over endoskeletons, particularly among small animals like spiders. They offer additional protections that endoskeletons do not, by covering the animal's entire body. Exoskeletons also allow for better leverage from the animal's muscles, compared to endoskeletons. This translates into more relative strength for their size when compared to animals with endoskeletons.