exoskeleton
Exoskeleton is a evolutionary innovation. This is operated by muscles attached to it and acts as an armor. Other than support this is an adaptive mechanism to provide protection against predators. In vertebrates, it is considered as the dermal bone that developed via membranous ossification. Fishes retained their exoskeleton in the form of their scales. However, in some organisms terrestrial evolution allows the loss of exoskeleton and formed a more mobile endoskeleton,