Zoology · Ch 2 — Kingdom Animalia
Coelom
Coelom
The coelom is the body cavity that lies between the body wall and the alimentary canal, and whether an animal has one — and how it is lined — is one of the clearest lines separating the animal phyla.
Acoelomates have no body cavity at all; the space between body wall and gut is packed solid with mesoderm-derived tissue, restricting the free movement of internal organs. Flatworms are the classic example.
Pseudocoelomates have a body cavity, but it is not fully lined by mesoderm — the mesoderm instead forms scattered pouches between ectoderm and endoderm, leaving a partially-lined cavity called a pseudocoel, filled with pseudocoelomic fluid that acts as a hydrostatic skeleton and allows the visceral organs to move and nutrients to circulate. Roundworms (Aschelminthes) are pseudocoelomates. …
What this figure shows. Three body-wall cross-sections placed side by side for comparison. The acoelomate section (e.g. flatworms) shows a solid body with no cavity: an ectoderm-derived outer body covering, an endoderm-derived digestive cavity lining, and the space between them completely packed with solid mesoderm-derived tissue, leaving no room for internal organs to move freely. The pseudocoelomate section (e.g. roundworms) shows an ectoderm-derived body covering, a mesoderm-derived muscle layer beneath it, then a fluid-filled pseudocoelom that is bounded by mesoderm on only one side, and an endoderm-derived digestive tract suspended loosely within that cavity. The eucoelomate section (e.g. earthworm) shows an ectoderm-derived body covering, then a true coelom fully enclosed on both sides by a mesoderm-derived lining (the peritoneum) that also suspends the endoderm-derived digestive tract by a double membrane, allowing the gut to move independently of the body wall — the arrangement that also distinguishes schizocoelomates (coelom forms by the mesoderm splitting, as in annelids, arthropods and molluscs) from entero …