What is an internal skeleton?

The internal skeleton is the passive part of the musculoskeletal system. Typically, it consists of bones, cartilage, joints, and ligaments.
Vertebrates (fish, frogs, lizards, birds, animals, humans) have an internal skeleton. Of invertebrates, some protozoa (radiolarians) and cephalopods have a primitive internal skeleton.
In vertebrates, the internal skeleton is formed by three sections: the skeleton of the head, the skeleton of the trunk, and the skeleton of the limbs. It is formed from bone or cartilage tissue. Sharks and rays have a cartilaginous skeleton throughout life. In most vertebrates, it is in the early stages of development of Cartilaginous, and with age it is almost completely replaced by bone.

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