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The development of a coelom was a major step in the evolution of the
animal body plan
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Its appearance in the evolutionary history of animals must
have brought with it a number of advantages considering that all annelids, molluscs,
arthropods, echinoderms and chordates are coelomate
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One
major disadvantage of having only a single body cavity is that muscle contractions,
such as those for locomotion, cause the gastrovascular cavity to become squeezed
and distorted
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Therefore acoelomate
organisms such as platyhelminthes, must rely on diffusion or on muscle contractions
for the transport of nutrients, respiratory gases and waste products around the body
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Formation of the coelom
Triploblastic animals develop from three germ layers – ectoderm, mesoderm and
endoderm
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Part of the mesoderm
forms a layer beneath the ectoderm while the other layer surrounds the endoderm
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Deuterostomes such as echinoderms and chordates develop their coelom differently
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Mesenteries attach the gut to the
body wall
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The increase in size would also add
tremendously to the weight of the animal
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An efficient circulatory system must also be
present and animals had to form new tissue for support
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In more complex organisms, internal organs bulge into the coelom which serves as a
protective buffer between the organs and the external environment
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It also permits space for development of sexual organs thus a large
number of gametes can be produced and provides a space for young to grow in
those animals which give birth to live young
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More power of locomotion
results as well in more variation in movement of both sets of muscles, permitting
different parts of the gut to become differentiated for different functions which
allows a greater variety of diets
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In some organisms with a fluid filled coelom
such as echinoderms, the fluid can circulate inside the body permitting the rapid
passage of materials from one part of the body to another and thus allowing larger
bodies to be developed
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This cavity originates differently than a true coelom
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Haemocoel
In arthropods and molluscs, the coelom is almost completely replaced during the
development by another cavity called the haemocoel
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The blood is
circulated in the haemocoel, and through several attached blood vessels
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Conclusion
The evolution of a coelomic cavity has truly brought along with it a great number of
advantages and has been one of the reasons behind the success of animals

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