What structure in vertebrates is mainly composed of bone and provides internal support and protection?

The structure in vertebrates that is mainly composed of bone and provides internal support and protection is the endoskeleton. This is a fundamental feature of vertebrate anatomy, as it serves several crucial functions. The endoskeleton not only supports the body's shape and structure but also protects vital organs—such as the heart and lungs housed in the thoracic cavity—by encasing them within bony structures, like the ribcage.

Furthermore, the endoskeleton plays a vital role in enabling movement by providing attachment points for muscles, facilitating locomotion through contraction. Composed primarily of bones, cartilage, and connective tissues, the endoskeleton also grows and adapts throughout an organism’s life, unlike external structures.

In contrast, the exoskeleton, which is found in arthropods and some mollusks, provides support from the outside and is made of chitin, not bone. The axial skeleton is a component of the endoskeleton that specifically includes the skull, vertebral column, and rib cage, but it does not encompass the entire internal support system. The pectoral girdle is another specific part of the skeleton, connecting the arms to the torso but again, it does not represent the whole endoskeleton. Thus