Skeleton
From Wikipedia, the free encyclopedia
- For other uses, see Skeleton (disambiguation).
In biology, the skeleton (from Greek σκελετός, "dried-up") 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.)
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[edit] Types and classification
Skeletal systems are commonly divided into three types - external (an exoskeleton), internal (an endoskeleton), and fluid based (a hydrostatic skeleton), although hydrostatic skeletal systems may be classified separately from the other two, since they lack hardened support structures.
[edit] External skeletons
Large external systems support proportionally less weight than endoskeletons of the same size, thus many larger animals, such as the vertebrates, have internal skeletal systems. Examples of exoskeletons are found in arthropods and shellfish, in which the skeleton forms a hard shell-like covering protecting the internal organs.
The phyla arthropoda and molluska both have exoskeletons. Since exoskeletons necessarily limit growth, phyla with exoskeletons have developed various solutions. Most mollusks have calcareous shells and, as they grow, the diameter of the shell is enlarged without altering its shape. On the other arm, arthropods shed their exoskeletons to grow, a process known as ecdysis (or molting). During molting the arthropod breaks down the old exoskeleton and then generates a new one, parts of which then harden, through various processes (such as calcification or sclerotization). An arthropod exoskeleton typically also has internal extensions, commonly referred to as an endoskeleton, although it is not a true endoskeleton.
[edit] Internal skeletons
An internal skeletal system consists of rigid (or semi-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, earthworms etc.), 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 thin to shorter and wider.
[edit] Skull classification
Amniotes, a group of animals that have an endoskeleton can also be further classed by their skeletons, specifically their skulls. The number of holes (temporal fenestra) in the top of their crania decide what class they fall into.
