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Skeletal Structures

Skeletal Structures

Leg bones (darkest gray), knee, muscles and tendons of a 44 year old man.
© Nuada Medical, Wellcome Images

  • Grades:
  • Length: 60 Minutes


Students design and build an exoskeleton or endoskeleton for an animal of their own invention.

This activity is from The Science of Muscles and Bones Teacher's Guide, and was designed for students in grades 6–8. Lessons from the guide may be used with other grade levels as deemed appropriate.

Teacher Background

Living things support and move their bodies against the pull of Earth’s gravity in many different ways. Tree trunks, lobster shells, floating lily pads and snake backbones all represent different solutions to this problem.

An animal’s support structure depends upon the size and shape of its body and also the environment in which it must live. Support structures can be inside (internal) or outside (external) of the body. External supports (exoskeletons) usually consist of hard plates or tubes that cover most or all of the body. Insects, spiders, clams and crabs all have exoskeletons. Exoskeletons protect internal organs, prevent water loss from the body surface and provide a protective shield from enemies/predators. However, since they encase the body, some kinds of exoskeletons must be shed and remade as an animal grows. Endoskeletons are located inside the body. Humans, mice, frogs, snakes, birds and fish all have endoskeletons. An endoskeleton grows along with the body but provides incomplete protection. Endoskeletons are living tissues that can have several functions. Some of these include storage of red bone marrow where red blood cells are made, storage of fat and minerals, and regulation of calcium distribution between bone and other tissue.

Most skeletons have one or more rigid sections connected at joints to allow movement. In endoskeletons, bones are connected across joints by tough fibrous ligaments. Muscles, which usually are attached to bones by tendons, make movement possible and also help support the body.

Objectives and Standards


  • Land animals and plants need support systems in order to stand and move against forces such as Earth’s gravity.

  • Skeletal systems, which can be inside or outside the body, provide support for animals.

Science, Health and Math Skills

  • Observing

  • Comparing

  • Modeling

Materials and Setup

Materials per Group of Student (see Setup below)

  • 15 paper clips

  • 10 straws

  • 2 sheets of card stock

  • Clay

  • Clear tape

  • Pair of scissors

  • Resealable plastic bag or plastic wrap (for skin or outer covering)

  • Ruler

  • Copy of the student sheet


  1. Place the plastic wrap, straws, clay, paper clips, tape, rulers, card stock and scissors in a central location.

  2. Have students work in groups of four.


Please follow all school district and school laboratory safety procedures. It always is a good idea to have students wash hands before and after any lab activity.

Procedure and Extensions

  1. Ask students to remember what happened to the plastic bag filled with water that they examined in the activity "Gravity and Buoyancy." Ask, Did the bag have the same shape in water as on the table? Students should be able to report that the bag was much flatter on the table. Follow by asking, Why don’t you and I flatten out on the floor, the way the bags did on the table? Use students’ answers to guide them into a discussion of support structures for living things, particularly animals. You might ask questions such as, Do all animals have some kind of support for their bodies? When present, what do we call these supports? (skeletons). Are all skeletons the same? How are skeletons different? (some are internal and some are external, some consist of many parts, others do not, some grow with the organism, others must be shed and replaced).

  2. After students have had opportunities to think about the variety of support structures for animal bodies, challenge them to invent an animal using the sheet as a guide. Depending on your students, you may want them to investigate different types of animal bodies using the World Wide Web or the library before they proceed further.

  3. Each group of students will need to decide where its animal lives and how it looks (especially body shape). Once groups have discussed their ideas, they should decide which type of skeleton (external or internal) would serve their animals best. Finally, each group should draw a design or plan for its animal. Encourage students to be creative. Show students the supplies (see materials list) that will be available for creating their animals OR ask students to make a list of materials to bring from home to build their animals.

  4. Once the groups’ plans are completed, have the Materials Managers collect straws, plastic bag/plastic wrap, tape, scissors, clay, paper clips, card stock and rulers for their groups.

  5. Have each group create its imaginary animal. Designate a time frame for this work.

  6. Ask groups to display their animals and to describe how they designed their skeletons.

  7. Draw a chart on the board with “Similarities” at the top of one column and “Differences” at the top of a second column. Ask the students to think about and discuss the similarities and differences of the various internal and external skeletons created by the groups.

  8. Extend the discussion by drawing two more charts on the board (see lesson PDF, sidebar, page 2 for lists of advantages and disadvantages of having an endoskeleton and an exoskeleton). Work with one chart at a time and ask students to respond.

  9. Conclude by asking students to share their ideas about how their animals might move. Ask, What allows us to move? What would we need to add to our animals so that they could move? Help students understand that, in most cases, muscles and joints are necessary, in addition to endo- or exoskeletons, to achieve movement of a body.

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National Space Biomedical Research Institute

National Space Biomedical Research Institute

This work was supported by National Space Biomedical Research Institute through NASA cooperative agreement NCC 9-58.