An exoskeleton is a rigid skeleton that lies outside the animal’s soft tissues. The first mineralized skeletons arose with sponges about 650 million years ago, but the more common exoskeletons subsequently evolved in other invertebrate groups adapted for life in aquatic and terrestrial environments (Fig. 37.16). Because they are on the exterior of the animal, exoskeletons provide hard external support and protection, allowing muscles to attach from the inside. However, a main disadvantage is that exoskeletons limit growth.

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Different invertebrates have different kinds of exoskeleton, but all types have relatively few cells for their volume of tissue. The shells of bivalve mollusks, such as clams and mussels, or the cephalopod mollusk Nautilus that has existed largely unchanged for nearly 500 million years, form a hard, mineralized calcium carbonate exoskeleton (Figs. 37.16a and 37.16b). The calcium carbonate is reinforced by proteins and is therefore an example of a composite material, one that combines substances with different properties. Because of its composite nature, the shell is less brittle and more difficult to break than if it were made of just one substance. The shell expands as the organism grows and epidermal cells deposit new layers of mineralized protein in regular geometric patterns. You can see the growth rings if you examine a mollusk shell. Although some marine bivalves achieve large size over many years, growth is limited by the rate at which new skeletal material can be deposited on the outside of the shell.

Arthropods have more complex exoskeletons. More than half of all known animal species, including insects and crustaceans, are arthropods. Arthropods are defined in part by their jointed legs, and another important feature is their exoskeleton formed of a cuticle that covers their entire body (Fig. 37.16c). Arthropod exoskeletons, which first evolved in aquatic crustaceans, protect animals from desiccation and physical insults. Consequently, exoskeletons were key to the success and diversification of insects in terrestrial environments about 450 million years ago.

The cuticle of insects is composed mainly of chitin, a nitrogen-containing polysaccharide. When initially formed, the cuticle of arthropods is soft, so the animal can grow before the cuticle hardens. Mature cuticle consists of two layers. A thin outer, waxy layer minimizes water loss. Water loss is especially dangerous for terrestrial arthropods, given their small size. The much thicker inner layer, whether flexible or stiff, is tough and hard to break. The cuticle remains flexible at joints, allowing motion between body segments. Because a rigid exoskeleton restricts growth, arthropods shed their cuticle at intervals, a process called molting. Molting allows arthropods to expand and grow before forming a new rigid exoskeleton.

Marine crustaceans, such as crabs and lobsters, incorporate calcium carbonate in their cuticle. Calcium carbonate makes the exoskeleton hard and stiff, much like the hardened cuticle of terrestrial insects, helping to protect these crustaceans from predators.

While offering several protective benefits, exoskeletons pose risks as well. Animals are vulnerable when the newly formed exoskeleton has not yet hardened. Exoskeletons are also hard to repair. If a skeleton is damaged while growing, the animal must produce an entirely new one. Because the exoskeleton is a thin-walled structure, it is prone to breaking if its surface area is very large. Consequently, the imperviousness of the monstrous insects depicted in science-fiction films is improbable—they would easily break from a blow to their exoskeleton.