Endoskeleton of a stingray

SKELETON ARTICULATED AND PHOTOGRAPHED BY STEVE HUSKEY, PH.D.

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NGSS: Core Idea: LS1.A, LS4.C

CCSS: Literacy in Science: 10

TEKS: 6.13B, 7.13A, 8.5F, B.3A

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Inside Out

On Halloween night, you might spot some spooky skeletons wandering around your neighborhood. Luckily, they’re just trick-or-treaters dressed in costumes. Real human skeletons are made of bones and found inside the body. But there’s more than one kind of skeleton. Some skeletons have no bones at all! Read on to learn about the four major types of skeletons.

On Halloween night, you might spot some spooky skeletons in your neighborhood. They’re just trick-or-treaters in costumes. Real human skeletons are made of bones inside the body. But there’s more than one kind of skeleton. Some skeletons have no bones at all! Read on to learn about four types of skeletons.

October 24, 2022
By Andrew Klein

AS YOU READ, THINK ABOUT how different types of skeletons help animals survive.

MADE FOR SWIMMING

Cartilaginous Endoskeleton

SKELETON ARTICULATED AND PHOTOGRAPHED BY STEVE HUSKEY, PH.D.

FEATHERY FINS: A stingray’s fins contain flexible structures, called ceratotrichia, that look like the bristles of a hairbrush. They allow rays to swim in a wave-like motion.

1. TOXIC TAIL: A barb at the end of a stingray’s tail delivers venom into a predator.

ALEX MUSTARD/NATUREPL.COM

SEA GLIDERS: A stingray swims along the seafloor, gobbling up tiny sea critters and fish with its mouth, found on the underside of its body.

Not all skeletons are made up of bones. Sharks, rays, and skates belong to a group of fish that have skeletons made of cartilage. That’s the same tough but flexible tissue that makes up your ears and the tip of your nose.

An endoskeleton, or internal skeleton, made of cartilage allows an animal’s body to bend easily. That makes it perfect for organisms that live in 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 Steve Huskey. He’s a biologist who studies animals’ skeletons at Western Kentucky University. A lightweight skeleton made of cartilage allows sharks and their close relatives to expend less energy than bony fish when moving in the water. That’s how they’re able to be such swift, agile swimmers.

Not all skeletons are made of bones. Sharks, rays, and skates are fish that have skeletons made of cartilage. That’s the same tough but flexible tissue that makes up your ears. It also forms the tip of your nose. 

This group of fish has an endoskeleton, or internal skeleton, made of cartilage. It allows an animal’s body to bend easily. It’s perfect for organisms that live in 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 Steve Huskey. He’s a biologist who studies animals’ skeletons at Western Kentucky University. A lightweight skeleton made of cartilage helps sharks and their relatives save energy. Compared with bony fish, they can move more easily in water. That helps them be fast swimmers.

SECRET SKELETON

Hydrostatic Skeleton

BLICKWINKEL/ALAMY STOCK PHOTO

DIGGING DEEP: An earthworm can burrow up to 6.5 feet into the ground.

Squishy animals like see-through jellyfish, tentacled sea anemones, or wriggly worms may not seem like they have skeletons. But they do—just not ones you can see. These creatures have hydrostatic skeletons. Their bodies have hollow cavities lined with muscles or other tough tissue. These spaces are filled with fluids under pressure.

For earthworms, “the liquid inside them pushes against the inside of their body to keep them in the shape of an earthworm,” says Huskey. “If all the liquid came out, they’d deflate into a flat tube.”

Hydrostatic skeletons also help animals like jellyfish and worms move their bodies. An earthworm, for example, squeezes and expands muscles starting at one end of its body and then down its entire length, explains Huskey. This wave-like motion is called peristalsis. As the worm’s muscles contract, they push fluid through the worm’s coelom, a tube that runs the length of its body. This propels the earthworm forward so it can creep through the soil.

See-through jellyfish, sea anemones, and wriggly worms are squishy. They may not seem like they have skeletons. They do—just not ones you can see. These creatures have hydrostatic skeletons. Their bodies have hollow spaces lined with muscles or other tough tissue. The spaces are filled with fluids.

For earthworms, “the liquid inside them pushes against the inside of their body to keep them in the shape of an earthworm,” says Huskey. “If all the liquid came out, they’d deflate into a flat tube.”

Hydrostatic skeletons help animals like jellyfish and worms move their bodies. An earthworm, for example, squeezes and expands muscles to move. The squeezing starts at one end of its body and then travels down its whole length, explains Huskey. This wavelike motion is called peristalsis. As the worm’s muscles contract, they push fluid through the worm’s coelom. That’s a tube that runs the length of its body. This pushes the earthworm forward so it can move through soil.

SUPPORT SYSTEM

Bony Endoskeleton

SKELETON ARTICULATED AND PHOTOGRAPHED BY STEVE HUSKEY, PH.D.

BOUNTY OF BONES: A snake has two ribs for each vertebra in its backbone.

Almost all vertebrates, or animals with a backbone, have a bony endoskeleton. That includes amphibians, birds, mammals, reptiles, and most fish. You fall into this category too. Without the support of an endoskeleton, your body would collapse into a blob, explains Huskey.

Almost all vertebrates, or animals with a backbone, have a bony endoskeleton. That includes amphibians, birds, mammals, reptiles, and most fish. You’re in this category too. Without the support of an endoskeleton, your body would collapse into a blob, explains Huskey.

PETER BLACKWELL/NATUREPL.COM

STRIKING A POSE: A red cobra can hold this defensive posture thanks to its flexible skeleton.

A bony endoskeleton serves several purposes. Because bones are hard and dense, they give the body both structure and strength. Some bones, like those that make up the skull and rib cage, provide protection for vital organs. And some bones work like factories, creating red blood cells inside the spongy marrow tissue found within their hard, dense exterior.

Many bones, like those in the legs and arms, are involved in movement. As muscles flex, they pull bones into different positions, moving limbs and other body parts. Bones connect at joints, which bend, allowing the bodies of organisms with bony endoskeletons to be flexible.

Different vertebrate species have varied numbers and arrangements of bones in their endoskeletons. While a giraffe’s neck can stretch to about 2 meters (6 feet) long, it is made up of only seven vertebra bones—the same number as in humans. And who is the winner for most bones in the animal kingdom? That award goes to enormous snakes called pythons, which can measure more than 9 m (30 ft) long, thanks to their approximately 1,800 bones.

A bony endoskeleton does many things. Bones are hard and dense. They give the body structure and strength. Some bones protect vital organs. The bones of the skull and rib cage do this. And some bones work like factories. They make red blood cells in their spongy marrow. This tissue is found inside the bones. 

Many bones are involved in movement. The bones or the arms and legs are two examples. As muscles flex, they pull bones into different positions. That moves limbs and other body parts. Bones connect at joints, which bend. Joints allow the bodies of organisms with bony endoskeletons to be flexible.

Different vertebrate species have different numbers and arrangements of bones in their skeletons. A giraffe’s neck can stretch to about 2 meters (6 feet) long. But it’s made up of just seven vertebra bones. That’s the same number as in a human’s neck. What has the most bones in the animal kingdom? Snakes called pythons top the list. They can measure more than 9 m (30 ft) long. And they have about 1,800 bones.  

BUILT-IN ARMOR

Exoskeleton

PIOTR NASKRECKI/MINDEN PICTURES

SKELETON UPGRADE: A whip scorpion in Costa Rica right after molting
1. molted exoskeleton; 2. scorpion with new soft exoskeleton

What do ants, clams, crabs, and spiders have in common? They all have exoskeletons, or skeletons on the outside of their bodies. In fact, many invertebrates, or animals without backbones, have this type of hard outer covering to protect their soft bodies.

Ancient organisms likely evolved the first exoskeletons by excreting hard mineral waste from food they ate onto the outside of their bodies. This formed a shell, says Huskey. It acted like armor, making it hard for predators to take a bite out of the animals. Today, the exoskeleton is the most common type of skeleton on Earth.

Because of their hard exteriors, animals with exoskeletons have a limited range of motion. The rigid outer covering also limits how much they can grow. As these animals get bigger, most must periodically molt, or shed their old exoskeleton. This reveals a larger exoskeleton. The new shell starts out soft and needs time to harden. “Animals are most vulnerable to attack when they are molting,” says Huskey. To avoid becoming a meal, an animal like a scorpion will hide under a rock for several days until its new exoskeleton has become tough enough to withstand an attack.

What do ants, clams, crabs, and spiders have in common? They all have exoskeletons. These are skeletons on the outside of their bodies. Many invertebrates, or animals without backbones, have this type of hard outer covering. It protects their soft bodies.

Ancient organisms likely evolved the first exoskeletons. They may have excreted hard mineral waste from food they ate onto the outside of their bodies. It formed a shell, says Huskey. It acted like armor. It made it hard for predators to take a bite out of these animals. Today, the exoskeleton is the most common type of skeleton on Earth.

Animals with exoskeletons have a hard exterior. So they have a limited range of motion. The rigid outer covering also limits how much they can grow. As these animals get bigger, most must periodically molt. That means they shed their old exoskeleton. This reveals a larger exoskeleton underneath. The new shell starts out soft. It needs time to harden. “Animals are most vulnerable to attack when they are molting,” says Huskey. To avoid becoming a meal, an animal like a scorpion will hide under a rock for several days. It will come out when its new exoskeleton has become tough enough to withstand an attack.

CONSTRUCTING EXPLANATIONS: In your own words, summarize the primary functions of a skeleton.

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