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Origin and Formation
Bilaterally symmetrical animals such as annelids, to which earthworms belong, have three tissue layers: endoderm, mesoderm and ectoderm. Such animals are eucoelomates, meaning they possess a true coelom, which originates from the mesoderm layer and is enclosed by it. The earthworm's body wall and gut are completely formed from mesoderm elements. Not all bilaterally symmetrical organisms possess a true coelom or body cavity. Pseudocoeloms, like nematodes, or roundworms, don't have it.
Eucoelomate Groupings
Eucoelomates have two categories: deuterostomes and protostomes. The main difference between the two is that during the early stages of development, deuterostomes form an anus first, while protostomes produce a mouth. Deuterostomes include annelids, mollusks and arthropods, while protostomes include chordates, echinoderms and hemichordates.
Coelom Functions
The coelom produces a hydrostatic skeleton. This skeleton maintains the segmented form of the earthworm, allowing it to burrow or move with coelomic compartment secretions that keep the earthworm's epidermis lubricated. The coelom also forms a circulatory system, which makes further growth possible. The coelom is where the organs are suspended, separating them from the gut. Hormones that the coelom produces make organ development and physiological functions possible.
Animals Without Coeloms
Acoelomates lack a coelom between the endodermic gut and the ectoderm; therefore, they possess no specialized vascular or circulatory systems. Pseudocoelomates possess a body cavity that is not lined with mesoderm components. They possess a simple excretory system and a hydrostatic skeleton supporting longitudinal musculature.