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Support from Skeletons and Bones

Skeletons give muscles something to pull against; support the body’s weight; store minerals like calcium and phosphorus; and produce blood cells in the bone marrow. However, not all animals have the same type of skeleton. Following are the three different kinds of skeletons you may see in your study of biology:

  • Hydrostatic skeletons are basically chambers filled with water. Animals with this type of skeleton move and change their shape by using muscles to squeeze their water-filled chambers — just like what happens when you squeeze a water balloon.

  • Exoskeletons are hard exterior coverings found on the outside of the body.

  • Endoskeletons exist within an animal’s body.

Animals with hydrostatic skeletons and exoskeletons are considered invertebrates, meaning they don’t have a backbone. Animals with endoskeletons, like you, are considered vertebrates because they have a backbone.

All vertebrate skeletons — whether they belong to humans, snakes, bats, or whales — developed from the same ancestral skeleton (which explains why you may notice similarities between your skeleton and that of your pet dog or cat).

Today, these animals show their relationship to one another in part due to homologous structures — structures that are equivalent to one another in their origin.

All vertebrate skeletons have two main parts (which you can see in the following figure):

  • The axial skeleton: This part supports the animal’s central column, or axis. It includes the skull, the backbone (vertebral column), and the rib cage.

  • The appendicular skeleton: This part extends from the axial skeleton out into the arms and legs (appendages). It includes the shoulders, the pelvis, and the bones of the arms and legs.

You should also know the names of some of your major bones:

  • Your skull protects your brain.

  • Your pectoral girdle includes your collarbones (clavicles) and your shoulder bones (scapulae).

  • Your sternum is the central bone in your chest, to which your ribs and collarbones attach.

  • Your vertebrae are the small bones in your back that protect your spinal cord.

  • Your ribs protect your lungs.

  • Your humerus is the long bone of your upper arm.

  • Your ulna is one of the two long bones in your forearm. It's on the same side of your arm as your little finger.

  • Your radius is the other long bone in your forearm. It's on the same side as your thumb.

  • Your carpals are the eight small bones that form your wrist.

  • Your metacarpals are the bones of your hand.

  • Your phalanges are your finger bones and toe bones. In your fingers, the phalanges are arranged in pairs in your thumb and in triplets in your fingers. In your toes, the phalanges are arranged in a pair in your big toes and in triplets in your other toes.

  • Your pelvic girdle includes your two hipbones and your tailbone.

  • Your femur, or the long bone of your thigh, is the longest bone in your body.

  • Your patella is your kneecap.

  • Your tibia is your shinbone.

  • Your fibula is the smaller bone in your leg that runs alongside your tibia.

  • Your tarsals are the seven small bones that make up your ankle.

  • Your metatarsals are the bones of your feet.

If you’ve ever watched an old Western movie, you’ve probably seen images of bones bleached white by the sun and scattered alongside a pioneer trail. These dry white bones are very different from the living bones in your body. Bone is actually a moist, living tissue that contains different layers and cell types.

  • Fibrous connective tissue covers the exterior of bones and helps heal bone breaks by forming new bone.

  • Bone cells (osteocytes) give bones their hard nature. The cells live in and produce a bone matrix made of collagen that’s been hardened by the attachment of calcium and phosphate crystals.

  • Cartilage covers the ends of bones and protects them from damage as they rub against one another.

The tissues found within living bone fall into two categories:

  • Spongy bone tissues are filled with little holes, similar to those you see in volcanic rocks. These holes are filled with red bone marrow, which is the tissue that produces your blood cells.

  • Compact bone tissues are hard and dense. A cavity within compact bone is filled with yellow bone marrow, which is mostly stored fat. If the body suddenly loses a large amount of blood, it converts the yellow bone marrow to red bone marrow so that blood cell production can be increased.

For questions 1–6, use the terms that follow to identify the type of skeleton found in the animal.

a. Hydrostatic skeleton

b. Exoskeleton

c. Endoskeleton

  1. A dog

  2. A beetle, like a ladybug

  3. An earthworm

  4. A shark (hint: sharks are cartilaginous fish)

  5. A bird

  6. State one advantage and one disadvantage for each type of skeleton.

  7. Print a copy of the following figure that will be used for questions 8–25. Using colored pencils or a highlighter, lightly shade the axial skeleton in the figure one color and shade the appendicular skeleton a different color.

For questions 8–25, use the terms that follow to label the bones in the figure.

a. Humerus

b. Skull

c. Metacarpals

d. Pelvic girdle

e. Vertebrae

f. Sternum

g. Phalanges of the hand

h. Fibula

i. Radius

j. Ulna

k. Femur

l. Tarsals

m. Tibia

n. Phalanges of the foot

o. Ribs

p. Metatarsals

q. Pectoral girdle

r. Carpals

[Credit:     From LifeART®, Super Anatomy 1, © 2002, Lippincott Willi
Credit:     From LifeART®, Super Anatomy 1, © 2002, Lippincott Williams & Wilkins

The following are the answers to the practice questions.

  1. The answer is c. Endoskeleton.

  2. The answer is b. Exoskeleton.

  3. The answer is a. Hydrostatic skeleton.

  4. The answer is c. Endoskeleton (but made of cartilage, not bone).

  5. The answer is c. Endoskeleton.

  6. Hydrostatic skeleton: Advantage is it’s very flexible; disadvantage is it’s not as good at resisting gravity as harder skeletons, and it doesn’t protect soft parts.

    Exoskeleton: Advantage is that it gives good protection to an animal’s soft parts; disadvantage is that it’s restrictive to growth. Animals with exoskeletons have to shed their skeletons in order to grow larger.

    Endoskeleton: Advantage is that it gives good support against gravity; disadvantage is that it doesn’t protect soft parts as well as exoskeletons.

  7. You should have shaded the skull, ribs, and spinal column in one color (axial) and the rest of the bones in another color (appendicular).

    The following is how the figure should be labeled:

  8. b. Skull

  9. e. Vertebrae

  10. q. Pectoral girdle

  11. o. Ribs

  12. f. Sternum

  13. a. Humerus

  14. i. Radius

  15. j. Ulna

  16. r. Carpals

  17. c. Metacarpals

  18. g. Phalanges of the hand

  19. d. Pelvic girdle

  20. k. Femur

  21. m. Tibia

  22. h. Fibula

  23. l. Tarsals

  24. p. Metatarsals

  25. n. Phalanges of the foot.

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