FIGURE 28–12 Muscles and Joints The diagrams show how muscles work with a vertebrate endoskeleton and an arthropod exoskeleton to bend and straighten joints. Compare and Contrast To what structure are arthropod muscles attached? To what structure are vertebrate muscles attached?

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Movement Arthropod muscles are attached to the inside of the exoskeleton. Vertebrate muscles are attached around the outside of bones. In both cases, different pairs or groups of muscles pull across joints in different directions. As you can see in Figure 28–12, when one muscle group contracts, it bends, or flexes, the joint. When the first group relaxes and the second group contracts, the joint straightens.

Vertebrate Muscular and Skeletal Systems An amazing variety of complex combinations of bones, muscle groups, and joints have evolved in vertebrates. In many fishes and snakes, muscles are arranged in blocks on opposite sides of the backbone. These muscle blocks contract in waves that travel down the body, bending it first to one side and then to the other. As these waves of movement travel down the body, they generate thrust. The limbs of many modern amphibians and reptiles stick out sideways from the body, as though the animals were doing push-ups. If you watch these animals move, you will see that many use sideways movements of their backbone to move their limbs forward and backward.

Most mammals stand with their legs straight under them, whether they walk on two legs or four. Mammalian limbs have evolved in ways that enable many different kinds of movement, as you can see in Figure 28–13 on the next page. The shapes and relative positions of bones and muscles, and the shapes of joints, are linked very closely to the functions they perform. Limbs that are specialized for high-speed running or long-distance jumping have very differently shaped bones, muscles, and joints than limbs adapted for flying, swimming, or manipulating objects. In fact, paleontologists can reconstruct the habits of extinct animals by studying the joints of fossil bones and the places where tendons and ligaments once attached.

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