∞ generated and posted on 2016.09.01 ∞
Coeloms are acellular volumes that are completely surrounded by body tissues and in particular are found within mesodermal tissue. These includes the cavities within which we find our hearts, lungs, and digestive systems.
In the following, very brief video, note the difference in coelom formation between protostomes and deusterostomes.
In certain animals, such as members of phylum Annelida, coeloms kept under pressure give rise to what are known as hydrostatic skeletons. Note that a body cavity is definitively not a gastrovascular cavity (though in certain animals the gastrovascular cavity, i.e., cnidarians, can indeed serve as a hydrostatic skeleton).
Members of phylum Nematoda possess instead what is known as a pseudocoelom since it their body cavity is not completely surrounded by mesodermal tissue. By contrast, members of phyla Cnidaria/I> and Platyhelminthes lack coeloms, with the former (Cnidaria) as diploblastic animals also lacking mesodermal tissue and the latter, which are triploblastic, are described as acoelomates.