Invertebrates Respond to “Breaking Symmetry” of Their Body Form Help

By — McGraw-Hill Professional
Updated on Aug 30, 2011

Introduction to Invertebrates Respond to "Breaking Symmetry" of Their Body Form

Symmetry, a rough balance in the shape and size of the body parts within an organism, is an important example of one kind of Biological Order. Thus, the bilateral symmetry of planaria worms, for example, as well as the radial symmetry of adult sea stars, are likewise models of biological balance and order.

What happens, then, when part of a planaria flatworm is cut off? Or what happens when one of the arms of a sea star is clipped? This type of disturbance is often called symmetry-breaking . By breaking of symmetry, it is meant that the rough balance between the parts of an organism has been disturbed. In humans, for example, amputating, say, the left leg below the knee, creates a severe symmetry-breaking. Because the left leg is now significantly shorter than the right leg, the normal bilateral symmetry of the human body has largely been broken. Without an artificial limb to correct this imbalance, the person’s ability to walk or even stand upright is severely compromised.

In sea stars and planaria flatworms, however, being relatively primitive invertebrates gives them certain advantages whenever their body symmetry is broken. Soon after a sea star loses an arm, for instance, the resulting breaking of its radial symmetry throws the animal into a state of severe imbalance as it tries to crawl over the ocean floor. This sudden introduction of great Biological Disorder strongly stimulates the cells within the stump of the missing arm to extensively grow and divide by mitosis. Due to the sea star’s amazing powers of regeneration, another arm soon grows to replace the one that was torn off. Unlike humans and other vertebrates, invertebrates like the sea star have no need for crutches!


Practice problems for these concepts can be found at: Invertebrates Test

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