In the past, and for practical purposes, coelom characteristics have been used to classify bilaterian animal phyla into informal groups.

Current hypothesis include:[citation needed][7] A coelom can absorb shock or provide a hydrostatic skeleton.

[clarification needed][9] The coelomic fluid serves several functions: it acts as a hydroskeleton; it allows free movement and growth of internal organs; it serves for transport of gases, nutrients and waste products around the body; it allows storage of sperm and eggs during maturation; and it acts as a reservoir for waste.

Animals were classified in three informal groups according to the type of body cavity they possess, in a non-taxonomic, utilitarian way, as the Acoelomata, Pseudocoelomata, and Coelomata.

The complete mesoderm lining allows organs to be attached to each other so that they can be suspended in a particular order while still being able to move freely within the cavity.

Tissue derived from mesoderm partly lines the fluid filled body cavity of these animals.

These protostomes have a fluid filled main body cavity unlined or partially lined with tissue derived from mesoderm.

This fluid-filled space surrounding the internal organs serves several functions like distribution of nutrients and removal of waste or supporting the body as a hydrostatic skeleton.

The coelom was apparently lost or reduced as a result of mutations in certain types of genes that affected early development.

These creatures do not have this need, as the surface area to volume ratio is large enough to allow absorption of nutrients and gas exchange by diffusion alone, due to dorso-ventral flattening.

A body cavity is also absent in placozoans, cnidarians (jellyfish and allies) and the ctenophores (comb jellies), but these animals are neither bilaterians or triploblastic.