Chitin
Chitin is a major constituent of the exoskeleton, or external skeleton, of many arthropods such as insects, spiders, and crustaceans. Exoskeletons made of this durable and firm compound support and protect the delicate soft tissues of these animals, which lack an internal skeleton. Chitin is a polysaccharide, a type of carbohydrate that has a basic structure of a repeating chain of sugar molecules. Chitin is analogous in structure to cellulose,the compound that provides structural support to plant tissues. In addition to being found in arthropod exoskeletons, chitin is also found in the cell walls of some species of fungi.
The exoskeleton of the Cayman Islands stone crab is made up, in part, of the polysaccharide chitin.Chitin does not work alone in forming exoskeletons. It is associated with a number of proteins, including an elastic, rubberlike substance called resilin. The identity and nature of these proteins determines whether the exoskeleton will be rigid, like a beetle's shell, or soft and flexible like the joints of a crab leg. Chitin also associates with nonprotein compounds, such as the calcium carbonate that is part of the shells of crustaceans such as crabs, lobsters, and shrimp.
Animals that wear their skeletons on the outside are relatively inflexible because of their armor rigidity. Arthropods can bend their limbs or the segments of their body only at the joints, where the exoskeleton is thinner. Therefore, it is important that the composition and character of the exoskeleton complement the anatomy it covers and the overall ecology of the organism.
Chitin confers a number of protective benefits to animals with exoskeletons. As well as defining the basic shape of the animal, the tough shell that encases arthropods protects the wearer from dessication, or dehydration. This particular function is essential to terrestrial arthropods, which may perish if too much water is lost from their blood and body tissues. Shells also provide effective protection against some predators.
Chitinous exoskeletons must be molted, or shed, as the animal grows because the rigid shell does not expand with the rest of the body. After the old shell is cast off, a new, larger exoskeleton is secreted by glands in the epidermis. Newly molted individuals are particularly vulnerable to attack because they have little protection while they wait for their new shells to harden.
Exoskeletons would be impractical for larger animals because chitin is not strong enough to protect and support them. Land-dwelling invertebrates, who do not benefit from the buoyant support of water, are limited in size because as an exoskeleton gets larger, it becomes thicker and heavier. These animals would not be able to move very well under the weight of this protective armor.
Bone; Keratin.
Bibliography
Anderson, D. T. Atlas of Invertebrate Anatomy. Portland: International. Specialized Bood Service, 1997.
"Chitin." The New Columbia Encyclopedia. Eds. William H. Harris and Judith S. Levy. New York: Columbia University Press, 1975.
Pierce, Sidney K., Timothy K. Maugle, and Lois Reid. Illustrated Invertebrate Anatomy: A Laboratory Guide. New York: Oxford University Press, 1989.
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