All animals have some form of a skeleton. Fish, amphibians, reptiles, birds, and mammals have an internal skeleton, made of bones or cartilage. Arthropods have a stiff internal skeleton. Worms and other soft-bodied invertebrates have a hydrostatic skeleton. The lesson in this chapter will compare exoskeletons and internal bony skeletons while also introducing the other main characteristics of arthropods. This lesson should be used to introduce the concept of invertebrates.