Wearing your skeleton outside means you have to crawl out of it
A crab or insect cannot simply grow bigger. Its skeleton is a rigid outer case, so to grow it must build a soft new one underneath, split the old one and climb out, then pump itself up before the new shell hardens. Some insecticides kill by jamming exactly this process, leaving the animal trapped in armour it has outgrown.
An exoskeleton is a hardened outer covering that both holds an animal's shape and shields its organs, the opposite of our own internal skeleton. Arthropods such as insects, spiders and crustaceans have one, as do tardigrades, corals and molluscs with their shells. It does more than protect: it can help with breathing, sensing, feeding and courtship, and on land it keeps the body from drying out. Contrary to a common belief, sea urchins and other echinoderms don't have one, because their shell-like test always sits inside living tissue.
Materials vary widely. Arthropod cuticle is built from chitin, stiffened with calcium carbonate at the cost of extra weight. Inward projections called apodemes anchor muscles; they are about six times stronger than vertebrate tendons and, in locusts, store elastic energy for jumping. Diatoms use silica, some tiny foraminifera glue sand grains onto themselves, and the scaly-foot snail armours itself with iron sulphide minerals. Outer shells have evolved many separate times, with 18 lineages arriving at calcified versions alone.
Hard parts are what fossilise, which is both a gift and a distortion. Shells survive when bodies rot, and muscle scars on the inside can reveal much of the anatomy. But two-thirds of the more than 30 living animal phyla have never been found as fossils at all, because they are soft-bodied. Mineralised skeletons appear shortly before the Cambrian, about 550 million years ago, and may have helped drive the Cambrian explosion.
Why armour appeared then is debated. A shift in ocean chemistry may have made shell minerals easier to deposit, but building the protein framework costs more than the mineral, so that alone seems insufficient. Shells arrived at about the same moment animals began burrowing to escape predators, and some of the earliest Cloudina shells bear drill holes, which points to an arms race.
Source: Exoskeleton
