Exoskeleton of cicada attached to Tridax procumbens. An exoskeleton (from Greek Î­Î¾Ï, Ã©xÅ outer and ÏÎºÎµÎ»ÎµÏÏÏ, skeletos skeleton) is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton (endoskeleton) of, for example, a human. In usage, some of the larger kinds of exoskeletons are known as shells.
The term arbitrary here refers to the fact that the symbol that people use is made or chosen without any principle, logic or reason. For example, people in Java do not have a principle or logic to call an animal with long tail, two hands, and two legs and like scratching and playing on trees as kethek.o, to answer Daniel's question: English people do not have any reason why they call the animal that produces meauwww as cat. Again, this is because language is arbitrary: no logical reason is used when choosing a symbol, and no direct relationship between the symbol and the symbolized.
In many cases, the ordinary equality symbol (=) is used in situations where, to be rigorous, the approximate equality sign ought to be used. This is done for two reasons: first, most fonts do not include an approximate equality symbol; and secondly, many people do not know what the approximate equality symbol means.