What does coelom mean?
The coelom is the fluid-filled body cavity found in many animals — from earthworms and insects to fishes and humans — that lies between the gut and the outer body wall and is fully lined with mesoderm, one of the three embryonic germ layers. This lining is what distinguishes a true coelom from simpler cavities like the pseudocoelom of roundworms, which lacks complete mesodermal covering. Functionally, the coelom suspends and protects internal organs, allows them to move independently of the body wall, and serves as a hydrostatic skeleton in soft-bodied creatures such as worms. In humans, parts of the embryonic coelom persist as the linings of the thoracic and abdominal cavities. The word derives from the Greek 'koilia', meaning belly or cavity, entering scientific English in the nineteenth century. It belongs firmly to zoology and comparative anatomy, where it plays a central role in classifying animal body plans as acoelomate, pseudocoelomate, or coelomate.
nounThe fluid-filled main body cavity of many animals, including vertebrates, that is lined with mesoderm and contains the digestive tract and other internal organs.
- The mesoderm-lined main body cavity in bilaterian animals such as annelids, molluscs, echinoderms, and chordates, which houses and cushions the internal organs.
"In the developing embryo, the coelom forms as a cavity within the mesoderm that will later house the abdominal organs."
"Earthworms have a well-developed coelom divided into segments by partitions called septa."
"Animals lacking a true coelom, such as flatworms, are described as acoelomates."
The plural follows regular English noun inflection; it appears mainly in comparative anatomy texts discussing multiple species or developmental stages.
"Different phyla exhibit coeloms of varying size and segmentation."
Every animal with a coelom — including you — is essentially a tube within a tube: your gut runs through a hollow chamber lined by tissue from the same embryonic layer that builds your muscles and bones.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of coelom
The word 'coelom' comes from the Ancient Greek 'koilōma', meaning 'cavity' or 'hollow', itself derived from 'koilos', meaning 'hollow'. It was adopted into scientific Latin and entered English usage in the nineteenth century, when comparative anatomists formalized the classification of animal body plans. The same Greek root gives us related terms such as 'coeliac' (pertaining to the abdomen) and 'coelacanth', the famous 'hollow-spined' fish, making these genuine cognates within biological nomenclature.
Related word forms
How coelom is actually used
Strictly technical vocabulary used almost exclusively in zoology, embryology, and comparative anatomy; it would be out of place in general prose. The adjective 'coelomic' and the agent nouns 'coelomate'/'acoelomate' carry the same register.
Easily confused with coelom
A coelacanth is a rare deep-sea fish, while a coelom is the fluid-filled body cavity found inside many animals; the two share only their Greek root for 'hollow'.