Earthworms have a hydroskeleton, birds have an endoskeleton and spiders have an exoskeleton. These are the three unique types of skeletal systems. This system is very important because it supports the body, safeguards the internal organs, and abets the mobility of living organisms.
We also have two broad classes of animals; vertebrates and invertebrates. Vertebrates have the endoskeleton while invertebrates have an exoskeleton and hydroskeleton or hydrostatic skeleton. The hydroskeleton is predominant in sea animals like starfish. A fluid-filled chamber in the body of these organisms forms the skeleton. The exoskeleton, also, is an exterior tough casing covering the body of an organism. It is predominant in insects (e.g., ladybug), arachnids (e.g., spiders), and arthropods (e.g., lobsters). The endoskeleton comprises interior rigid calcified organs called bones.
Furthermore, the endoskeleton performs two additional functions namely; to help in mineral storage and in the production of blood cells