An endoskeleton is a bone or cartilage-based skeleton that exists wholly inside of an animal’s body. All vertebrates, including humans, have endoskeletons. Echinoderms — a class that includes starfish and some sea anemones — also have this sort of bone structure. It is characterized by a spine from which bones extend, protecting the animal’s internal organs from within.
While never visible on an animal from the outside, the endoskeleton is nevertheless an essential aspect of animal anatomy. It is completely contained within the body, and grows as the animal grows. This makes it different from an exoskeleton in several respects.
Many insects and crustaceans have exoskeletons, which are tough, shell-like structures that cover the body from the outside. These structures are static, which means that they do not grow. Animals with exoskeletons either remain at a constant size throughout their lives or molt their old exoskeletons in order to generate entirely new ones as they grow.
In contrast, endoskeletons are permanent parts of vertebrates’ bodies. The endoskeleton begins developing in the embryonic stage. Baby animals’ bones are often made of cartilage at first, then turn to bone over time through a process known as ossification. As the animal grows, the bones strengthen, thicken, and elongate until they reach full size.
The skeletal system of vertebrates is characterized by several easily identifiable parts. First is the spine. All endoskeletons are built around a stacked spine of joined discs formed as a column that houses the animal’s central nervous system.
At the top of the spine is a skull, which houses the brain. The only exception to this rule occurs with echinoderms. Echinoderms do not have skulls or brains. Their movements are controlled wholly by their central nervous system.
Limbs, fins, and any other extremities also extend from the spine. In most animals, the endoskeleton is covered in muscles, ligaments, and tissues. These coverings allow the endoskeleton to play a major role in body movement and motor control. The bone structure afforded by the endoskeleton allows the body to stand, sit, bend, and swim with precision.
Organ protection is an equally important endoskeleton function. Vertebrates’ bodies are regulated by an intricate system of internal organs, including hearts, lungs, kidneys, and livers. The endoskeleton protects these organs from damage by shielding them with a “cage” of rib bones.
Echinoderms are again an exception, as these animals largely lack internal organs. Their internal structure is usually based around a system of hydraulic chambers and canals, which move fluids and nutrients throughout the body. These internal chambers resemble the inner workings of a hydrostatic skeleton, which is a flexible, fluid-filled cavity that helps soft-bodied animals like earthworms move. The main difference between an echinoderm’s skeleton and a hydroskeleton is the calcified bone structure.