In vertebrates, an exoskeleton is a rigid external covering that supports and protects the animal’s body. The exoskeleton of arthropods, such as insects and crabs, is composed of chitin. In crustaceans, the exoskeleton may also be calcified. Exoskeletons have evolved multiple times in different taxa; they first appeared among animals in the Cambrian period about 540 million years ago and since then have been widely adopted to enable various lifestyles.
There are three main types of exoskeletons: proteinaceous (e.g., arthropods), mineralized (e.g., mollusks), and composite (e.g., some vertebrates). Exoskeletons provide structure for muscles to attach to, protection from predators and the environment, support for internal organs, storage for energy reserves, sensory surfaces for information gathering about the environment, and a surface for gas exchange with the environment. They may also serve as weapons or mating displays.
Proteinaceous exoskeletons are composed of proteins such as collagen and fibrous proteins that are cross-linked by covalent bonds to form a tough matrix surrounding the animal’s body. These cross-links make proteinaceous exoskeletons extremely strong relative to their weight and resistant to deformation. However, because they are organic materials, proteinaceous exoskeletons can degrade over time if not properly cared for or replaced. Arthropods shed their old cuticle periodically during molting cycles and replace it with a new one that has been grown underneath; this process is called ecdysis.
Minerals can be deposited in an organism’s extracellular matrix or within cells to produce a mineralized exoskeleton . Mollusks , segmented worms , echinoderms , hemichordates , some poriferans , and some tunicates produce mineralized exoskeletons made mostly of calcium carbonate crystals . The resulting skeletons are harder and more durable than proteinaceous ones but weigh more due to their higher density . While providing better protection from predators , physical damage , dehydration ,and infection than non-mineralized skeletons , mineralized skeletons limit an animal’s ability to grow by restricting how much water can be taken in . As a result , most animals with mineralized skeletons must shed them periodically throughout their lives so that they can grow . This process is called molting or shedding .
Some animals have both proteinaceous and mineralized skeletal elements in what is known as a composite skeleton . For example, many sharks have tooth plates made of dentine surrounded by enamel that covers part of their scale -like dermal tissue ; these structures give added strength without significantly increasing weight Vertebrates typically have endochondral bones —that is bones that develop within cartilage—surrounded by periosteum In contrast , invertebrate chordates generally have an unsegmented notochord surrounded by thin sheets of muscle ; this provides adequate support without being too heavy Other animals combine features of both kinds of skeleton s depending on their needs For example shrimp – like krill living in open water often have light transparent shells while those living at greater depths where predators abound tend towards heavier less transparent shells Wood lice roll into balls when disturbed which protects them from drying out but makes them more vulnerable to attack so they only do this when necessary Some deep sea fish generate hydrostatic pressure equal to several atmospheres through muscular contraction which gives them enough support to move about despite having very little bone Invertebrate chordates use external structures such as spines or fin rays rather than bony internal ones whenever possible because these require less energy to maintain