Animal Skeletons and Exoskeletons思维导图

Turn to page 10. Why do animals have skeletons inside their body? You see, not all animals have skeletons, but most large ones do. This is because the bigger an animal is, the more it needs a strong sturdy framework to hold its body together and carry its weight. Skeletons also protect soft inside parts, like brains and hearts. Which animals have skeletons on the outside? Most smaller animals have tough skins called exoskeletons. These outside skeletons do the same job as inside ones. They protect and support the animals' soft bodies. Animals without backbones are called invertebrates. Insects, spiders, snails, warms, jellyfish, prawns and crabs are all invertebrates. Animals with backbones are called vertebrates. Fish are vertebrates and so are amphibians, reptiles, birds and mammals. These animals' skeletons are made of bone, but a shark's skeleton is made of Bristol. This isn't as hard as bone, but it's still tough. You have some at your no-stip. Insects, spiders, scorpions, centipedes and millipedes all have tough exoskeletons. Obsters, crabs and some beetles have really tough exoskeletons that work like armor to protect them from attack. Squid are the biggest kind of invertebrate. The longest one ever found measured more than 17 meters from its head to the tip of its tentacles, longer than eight scuba divers. To grow larger, an animal has to break out of its old exoskeleton and grow a new one.