Skeleton
From Wikinfo
Skeleton is also a winter sport: see skeleton (sport).
In biology, the skeleton or skeletal system is the biological system providing 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 two types - external (an exoskeleton), and internal (an endoskeleton). The third skeletal system is called the hydrostatic skeleton which is hardly ever mentioned when speaking of the skeletal system because it lacks bones.
External skeletal systems are restricted in their maximum size so larger animals, such as the order chordata, have internal skeletal systems. Examples of this are found in arthropods and shellfish: the skeleton forms a hard shell-like covering protecting the internal organs.
The phylum arthropoda and mollusca have exoskeleton. In terms of growing its body, each phylum has its unique process. Most molluscs have calcareous shell that covers and protects their bodies. When they grow, the diameter the shell is enlarged without altering its coiled shape. The outer edge of the shell is added to its original one.
On the other hands, the arthropods must shed their exoskeleton to grow. At first, they swell inside their exoskeleton. As a certain period of time passed, the outer covering splits and the inner part of the body comes out. The process of shedding the exoskeleton is called molting. After that, the body quickly makes a new larger covering. In order to harden the skeleton, crustaceans such as crabs and lobsters deposit calcium salts into their cuticles that secreted by the epidermis.
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.
The fully-developed human skeleton consists of 206 bones.
The hydrostatic skeleton is sort of like a water-filled balloon. Located internally in cnidarians and annelids, these animals can move by contracting its muscles in the wall of the fluid-filled pouch which creates pressure within the pouch which creates movements. These animals use the hydrostatic skeleton to change the shape of their body from long and skinny to short and stumpy.
References
- Adapted from the Wikipedia article, "Skeleton" http://en.wikipedia.org/wiki/Skeleton December 2, 2003
