Exoskeleton
From Palaeos
An exoskeleton is an external anatomical feature that supports and protects an animal's body, in contrast to the internal endoskeleton of vertebrates. Whilst many invertebrates (such as shelled mollusks) have exoskeletons in the sense of external hard parts, the character is most associated with the arthropods. Exoskeletons contain rigid and resistant components that fulfil a set of functional roles including protection, excretion, sensing, support, feeding and (for terrestrial organisms) acting as a barrier against desiccation. Exoskeletons first appeared in the fossil record about 550 million years ago, and their evolution has been seen as a critical factor in the Cambrian explosion of animals.
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[edit] Arthropod exoskeletons
[edit] Microscopic structure
A typical arthropod exoskeleton is a multi-layered structure with four functional regions: epicuticle, procuticle, epidermis and basement membrane [1]. Of these, the epicuticle is a multi-layered external barrier that, especially in terrestrial arthropods, acts as a barrier against desiccation. The strength of the exoskeleton is provided by the underlying procuticle, which is in turn secreted by the epidermis. Arthropod cuticle is a biological composite material, consisting of two main portions: fibrous chains of alpha-chitin within a matrix of silk-like and globular proteins, of which the most well-known is the rubbery protein called resilin. The relative abundance of these two main components varies from approximately 50/50 to 70/30 protein/chitin, with softer parts of the exoskeleton having a higher proportion of chitin. Although the cuticle is relatively soft when first secreted, it soon hardens in a poorly-understood process that involves dehydration and/or tanning mediated by hydrophobic chemicals called phenolics. Different types of interaction between the proteins and chitin leads to varying mechanical properties of the exoskeleton.
In addition to the chitino-proteinaceous composite of the cuticle, many crustaceans, some myriapods and the extinct trilobites further impregnate the cuticle with mineral salts, mainly calcium carbonate, which can make up up to 40% of the cuticle. This can lead to great mechanical strength.
[edit] Organisation
The arthropod exoskeleton is typically divided into different functional units to allow flexibility in an often otherwise rigid structure. For example, the head is a fused capsule, and the trunk is often divided into a series of articulating sclerites called tergites. In addition, the characteristic limbs of arthropods are jointed. The internal surface of the exoskeleton is often elaborated into a set of specialised structures called apodemes that allow the attachment of muscles. Such endoskeletal components of the arthropod skeleton can be highly complex, as in crabs and lobsters.
[edit] Ecdysis
The relative rigidity of the exoskeleton means that continuous growth of arthropods is not possible. Therefore, growth is periodic and concentrated into a period of time when the exoskeleton is shed, called moulting or ecdysis, which is under the control of a hormone called ecdysone. Moulting is a complex process that is invariably dangerous for the arthropod involved. Before the old exoskeleton is shed, the cuticle separates from the epidermis through a process called apolysis. New cuticle is excreted by the underlying epidermis, and mineral salts are usually withdrawn from the old cuticle for re-use. After the old cuticle is shed, the arthropod typically pumps up its body (for example, by air or water intake) to allow the new cuticle to expand to a larger size: the process of hardening by dehydration of the cuticle then takes place. Newly molted arthropods typically appear pale or white, and darken as the cuticle hardens.
[edit] Natural exoskeletons
Excellent as defensive adaptation, exoskeletons may nevertheless cause problems where entities carry an excessive weight to surface-area ratio or whenever organism growth requires an enlargement of the exoskeleton. Exoskeletons are found on many different types of organisms including shellfish and insects.
[edit] Types of Exoskeleton and Representative Organisms
- Bone and cartilage
- Ostracoderms (extinct armoured fish)
- Testudines (turtles, terrapins and tortoises)
- Chitin
- Arthropods
- some Bacteria
- Calcium compounds
- Silicate
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