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Elementary Science Education

Three distinct body designs that enable movement


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      Skeleton inside muscles outside (Vertebrates)

      Have students reflect on the idea that vertebrates illustrate one basic body design, namely a skeleton on the inside–typified by a back bone–moved by muscles on the outside. Discuss: Are there other possible ways of designing a body? What about putting a skeleton on the outside and operate it with muscles on the inside? What about doing away with the skeleton altogether and enabling the animal to simply “squeeze and squish” its way from one place to another? In fact, we find numerous animals in nature that illustrate both of these alternatives. Challenge students to think of what they are.

      Skeleton outside muscles inside (Arthropods)

      Examples of animals with skeletons on the outside are: insects, spiders, crabs, lobsters, and many others as may be seen in these photos:

      Type into your browser: arthropod images

      This feature of having the skeleton on the outside and many jointed appendages leads scientists to classify such animals a arthropods meaning “many feet”.

      With the skeleton on the outside, how do joints of arthropods function? The outside skeleton (exoskeleton) is hollow. Muscles inside attach to points across a joint. Contraction of one muscle pulls the leg one way; contraction of the alternate muscle pulls it the other way. You can see illustrations by typing into your browser:

      arthropod joint and muscles illustration

      Have students note the similarity in concept to movement of vertebrate joints?

      A problem in having the skeleton on the outside is: How does the animal grow? The exoskeleton is rigid and can’t stretch. In fact, to grow the animal must “climb” out of its old exoskeleton, expand, and then grow a new exoskeleton as is seen in this video of a molting crab.

      No skeleton (worms, jelly fish, octopuses, and others) (several phyla)

      Worms have two sets of muscles: circular muscles that are like a series of belts around the worm from its “head” to its “tail”, and a set of longitudinal muscles that run along the length of the worm. Observe in the following video how alternate contraction of these muscles squeezes the front forward, then pulls up the back. (It has tiny barbs in the front portion of its underside. It uses these to grasp and hold the front portion while pulling the rear. If you don’t mind holding the worm, you can feel these barbs. (Please post question/comments)

      Classification of animals

      It is such differences in body design that lead scientist to make the first step in classification, i.e., separating animals into major groups called phylums. All those with a skeleton (back bone) on the inside are vertebrates; those with an exoskeleton are arthropods. There are several phylums of animals without skeletons based on additional differences in in body design.  (Please post questions and comments.)



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