Skeleton system of vertebrates and invertebrates.

Skeleton system of vertebrates

In vertebrates the adult skeleton is usually formed of bone or cartilage—living substances that grow with the animal, in contrast to the many types of the invertebrate skeleton that do not grow or are dead secretions, deposits, or crystals. The internal position of bones and their central position in limbs provide firm support for small and large animals. Muscles can be inserted on all surfaces of the skeleton, in contrast to the limitations of the cuticular skeleton of arthropods, in which muscles occur on only one side. Antagonistic muscles are easily placed upon vertebrate bones to allow contrasting movements at the joints between them.

Invertebrate skeletons may be hydrostatic or stiff, either type serving to support the body and keep it in shape; stiff skeletons usually also have protective functions. All animals have skeletons of some kind, both at the cellular level (the cytoskeleton) and at the organism level.

There are two major invertebrate versions of stiff jointed skeletons, the exoskeletons in arthropods and the endoskeletons in echinoderms. The arthropod exoskeleton is formed from a complex cuticle, with three main layers secreted from the epidermis.