Evolutionary significance of Coelom in Annelids

Formation of coelom in animals is a giant leap in the evolutionary history of Kingdom animalia. Coelom is a fluid filled space between body wall and visceral organs, lined by mesodermal epithelium called 'Peritoneum'. Based on the coelom, triploblastic metazoans are categorized into 3 groups viz., Acoelomata, Pseudocoelomata and Eucoelomata.
Acoelomates are the group of animals in which coelomic space is completely filled with dense fibrous mesenchyme tissue derived from mesoderm. Hence they are without coelom and called Acoelomates. Ex. Phylum Platyhelminthes.
Pseudocoelomates are the group of animals in which mesodermal layer is occupying only adjoining areas of the body wall. Remaining space is filled with Pseudocoelomic fluid. As the coelom is not lined completely by mesodermal peritoneum, this type of coelom is called as Pseudocoelom. Ex. Phylum Nematoda
Eucoelomates are the group of animals in which mesodermal epithelium completely lines the coelom, hence call true coelom. Coelom is filled with Coelomic fluid. Ex. From Phylum Annelida to Phylum Chordata.

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