Endoskeletons.

The study of endoskeletons can be approached in three basic ways. First, endoskeletons can be approached as gross structures—that is, as primarily mechanical or architectural systems. Second, they can be viewed as types of tissue—bone and cartilage—which involves study at the microscopic level. Third, endoskeletons can be considered products of the evolution of the endoskeletal organism. This area examines such factors as the organisms’ development from protochordates and the intrinsic biological advantages of the endoskeleton over the exoskeleton.