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STANDARDS
NGSS: Core Idea: LS4.C: Adaptation
CCSS: Reading Informational Texts
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Spooky Skeletons
Find out about the four different types of skeletons of the animal kingdom.
SUPPORT SYSTEM
SKELETON ARTICULATED AND PHOTOGRAPHED BY STEVE HUSKEY, PH.D.
CHOMP! An alligator’s skull protects its brain. It uses its jaws to bite into food. TURBO TAIL: An alligator swishes its tail side to side so it can swim fast.
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AMERICAN ALLIGATOR: These reptiles can grow to 15 feet long and weigh 1,000 pounds.
Animals with skeletons inside of their bodies have endoskeletons. Organisms like amphibians, birds, mammals, reptiles, and most fish have endoskeletons. You fall into this category too! Without the support of your endoskeleton, your body would collapse into a blob, explains Steve Huskey. He’s a biologist at Western Kentucky University who studies skeletons.
An endoskeleton helps you in several ways. Some of the hard bones provide protection for vital organs. Others, like those in the legs and arms, are involved in movement. As muscles flex, they pull bones into different positions. This allows animals to move limbs and other body parts. Bones connect at joints. These structures allow you to be flexible so you can do a silly—or spooky—dance!
BUILT-IN ARMOR
BLACK SCORPION: This scorpion molts about five times during its life span—which is about eight years.
What do ants, clams, crabs, and spiders have in common? They all have exoskeletons. This type of skeleton is a hard outer covering that protects an animal’s soft body inside.
However, the hard coverings limit how much the animals can grow. As they get bigger, some animals must molt. This reveals a new, soft exoskeleton that needs time to harden. “Animals are most vulnerable to attack when they are molting,” says Huskey. Organisms, like scorpions, will hide until their armor hardens.
MADE FOR SWIMMING
YELLOW STINGRAY: A stingray’s fins are filled with flexible structures that look like the bristles of a hairbrush. They help rays swim in a wavelike motion.
Animals like sharks and rays belong to a group of fish with endoskeletons. But their internal skeletons aren’t made of hard bones. Their skeletons are made of cartilage. It’s the same tough but flexible tissue that makes up the tip of your nose and ears.
A skeleton made of cartilage allows an animal’s body to bend easily. That makes it perfect for creatures that live in the water. For example, rays glide through the ocean by waving their fins up and down, like wings.
Cartilage also weighs less than bone. “When you pick up a stingray’s skeleton, you barely even know you’re holding it,” says Huskey. The lightweight skeleton lets these animals swim fast.
SECRET SKELETON
ISTOCKPHOTO/GETTY IMAGES
DIGGING DEEP: An earthworm can burrow down more than 6 feet below the surface.
Squishy animals like clear jellyfish or wriggly worms may not seem like they have skeletons. They do—just not ones you can see. These creatures have hydrostatic (hye-druh-STA-tick) skeletons. They have hollow spaces inside their bodies. These spaces are filled with fluids. “The liquid inside an earthworm pushes against the inside of its body to keep it in the shape of an earthworm,” says Huskey. “If all the liquid came out, it would deflate into a flat tube.”
An earthworm also uses its hydrostatic skeleton to crawl. It squeezes muscles that push fluid through its body so it can wiggle through the soil.
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