The three main types of a skeleton are the hydrostatic skeleton, the exoskeleton, and the endoskeleton.

a. Hydrostatic skeleton: It is made up of fluids contained in a body cavity and it allows the organism to change its body shape by contracting its body walls.

b. Exoskeleton: It is made up of chitin or calcium carbonate and serves as the organism’s outer body covering.

c. Endoskeleton: It is made up of calcified plates, bones, or cartilage that gives structural support to the body of organisms.

Animals have muscles that work together in pairs or groups and they are usually attached to the skeleton, particularly in the joints. When the muscles pull the skeleton by contraction, the body generates a movement.

An animal with an exoskeleton is vulnerable to attacks during molting, which is a process of breaking out the exoskeleton in order to grow a new one. Once the animal makes it out of its exoskeleton, they become defenseless from predators.

The larger the organism becomes, the heavier its exoskeleton gets. This indicates that an extra large exoskeleton would weigh extra heavy. In this case, if a spider grows too large, it would not have the power or strength to move around. This is the reason why their growth is limited to a small size only.

The illustration below shows the biceps and triceps muscles that are attached to the elbow joint of humans. When these muscles work together, the elbow would either bend or straighten. When the elbow bends, the biceps muscle (flexor) contracts and the triceps muscle (extensor) relaxes.

The illustration below shows the biceps and triceps muscles that are attached to the elbow joint of humans. When these muscles work together, the elbow would either bend or straighten. When the elbow bends, the biceps muscle (flexor) contracts and the triceps muscle (extensor) relaxes.