Skeletons: Exoskeletons, Endoskeletons, and Hydrostatic Systems

Skeletons: Types and Functions

Exoskeletons

External systems support proportionally less weight than endoskeletons of the same size, which is why larger animals such as vertebrates have internal skeletal systems.

Key developments in exoskeletons are found among arthropods and some invertebrates, where the exoskeleton forms a shell or outer structure that protects the internal organs.

Given the limitations on animal growth imposed by exoskeletons, species with this feature have evolved various solutions. Most molluscs have calcareous shells that grow along with the animal, maintaining a consistent growth morphology. Other animals, such as arthropods, shed their old exoskeleton to grow, a process known as "molting." The new exoskeleton is hardened by calcification and ossification processes.

The exoskeleton of an arthropod often presents internal extensions, known as endoskeletal structures, although they are technically part of the exoskeleton.

Internal Skeleton (Endoskeleton)

An internal skeleton consists of rigid or semi-rigid structures inside the body, moved by the muscular system. If such structures are mineralized or ossified, as in humans and other mammals, they are called bones. Another component of the skeletal system is cartilage, which complements the structure. In humans, for example, the nose and ears are supported by cartilage. Some organisms have a skeleton consisting entirely of cartilage and without any calcified bones, as in the case of sharks. Bones and other rigid structures are connected by ligaments, and muscles attach to the system through tendons.

The internal skeleton, or endoskeleton, is a structure connected by bones, forming a sturdy frame with joints.

Fluid or Hydrostatic Skeleton

The hydrostatic skeleton consists of a fluid-filled cavity, either celomatic or pseudocelomática, surrounded by muscles. The fluid pressure and the action of surrounding muscles are used to reshape the body and produce movement, such as digging or swimming. The subsequent contraction of different metameres, which are provided with bundles of circular and longitudinal muscle fibers, stretches and swells body parts, allowing for horizontal movement. Hydrostatic skeletons play a role in the locomotion of echinoderms (starfish, sea urchins), annelids, nematodes, and other invertebrates. The hydrostatic muscles share similarities with hydrostatic skeletons.

It is characteristic of coelomate organisms such as annelids. These animals can move by contracting the muscles surrounding the fluid bag, creating pressure inside the pouch that causes movement. Some earthworms use their hydrostatic skeleton to change shape as they move forward, dilating and shrinking their bodies.