Skeleton
In
biology, the
skeleton or
skeletal system is the biological system providing physical support in living organisms. (By extension, non-biological outline structures such as
gantries or
buildings may also acquire skeletons.)
Skeletal systems are commonly divided into three types - external (an
exoskeleton), internal (an
endoskeleton), and fluid based (a
hydrostatic skeleton), though hydrostatic skeletal systems may be classified separately from the other two since they lack hardened support structures.
External skeletons
Large external skeletal systems support proportionally less weight than endoskeletons of the same size, and thus many larger animals, such as the
vertebrates, have internal skeletal systems. Examples of exoskeletons are found in
arthropods,
shellfish, and some insects: the skeleton forms a hard
shell-like covering protecting the internal organs.
The
phyla arthropoda and
mollusca both have exoskeletons. Since exoskeletons necessarily limit growth, phyla with exoskeletons have come up with various solutions. Most molluscs have calcareous shells and as they grow, the diameter of the shell is enlarged without altering its shape. On the other hand, arthropods shed their exoskeletons to grow, a process known as
ecdysis (or moulting). During molting the arthropod breaks down their old exoskeleton and then regenerates a new one, parts of which they then harden, through various processes (such as calcification or
sclerotization). An arthropod exoskeleton typically also has internal extensions, commonly referred to as an
endoskeleton, though it is not a true endoskeleton.
Internal skeletons
An internal skeletal system consists of rigid structures, within the body, moved by the
muscular system. If the structures are mineralized or
ossified, as they are in humans and other mammals, they are referred to as
bones.
Cartilage is another common component of skeletal systems, supporting and supplementing the skeleton. The human
ear and
nose are shaped by cartilage. Some organisms have a skeleton consisting entirely of cartilage and without any calcified bones at all, for example
sharks. The bones or other rigid structures are connected by
ligaments and connected to the muscular system via
tendons.
Hydrostatic skeletons are similar to a water-filled balloon. Located internally in
cnidarians (
coral,
jellyfish, etc.) and
annelids (
leeches), among others, these animals can move by contracting the muscles surrounding the fluid-filled pouch, creating pressure within the pouch that causes movement. Animals such as earthworms use their hydrostatic skeletons to change their body shape as they move forward from long and skinny to short and stumpy.
Skull classification
Amniotes, a group of animals that have an endoskeleton can also be further classed by their skeleton, specifically their
skull. The number of holes (
temporal fenestra) in the top of their cranium decides what class they fall under.
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Human skeleton
