Biology · Animal Kingdom · NEET
A coelom is a fluid-filled space (cavity) inside the body, found between the body wall on the outside and the gut (alimentary canal) on the inside. In a true coelom, this space is completely lined by mesoderm, the middle germ layer. Animals that have this true, mesoderm-lined cavity are called coelomates, such as annelids, molluscs, arthropods, echinoderms, hemichordates and chordates.
No, and this is where students lose marks. Every space is a 'body cavity', but NCERT calls it a true coelom only when the cavity is lined by mesoderm on all sides. If mesoderm is present only as scattered pouches (not a full lining), it is a pseudocoelom. If there is no cavity between the body wall and gut at all, the animal is acoelomate. So the word 'coelom' is about the mesoderm lining, not just an empty space.
Mesoderm, the middle of the three germ layers (ectoderm, mesoderm, endoderm). In a true coelomate, mesoderm forms a complete lining around the body cavity, touching both the body wall and the gut wall. In a pseudocoelomate, mesoderm is present only as scattered pouches between the ectoderm and endoderm, so the cavity is not fully lined. NEET often shows this exact difference in a diagram-style question.
Because the presence, absence, or type of coelom is a stable, fundamental feature that separates major animal groups. NCERT lists 'nature of coelom' along with symmetry, germ layers and level of organisation as a basis of classification. For example, Platyhelminthes are acoelomate, Aschelminthes are pseudocoelomate, and Annelida onwards are true coelomates. Knowing the coelom type instantly tells you which phylum an animal can belong to.
No. The gut (alimentary canal) is the tube where food is digested. The coelom is the separate space between the body wall and this gut wall. A true coelom lets the gut hang freely and move independently of the body wall. Do not confuse the hollow inside of the gut with the coelom.
Bilaterally symmetrical and acoelomate animals are exemplified by
Consider the following statements: A. Annelids are true coelomates. B. Poriferans are pseudocoelomates. C. Aschelminthes are acoelomates. D. Platyhelminthes are pseudocoelomates. Choose the correct answer.
A researcher did histology of an adult animal and observed a cavity with mesodermal tissue towards the body wall but no mesodermal tissue towards the alimentary canal. What could be the possible coelom of that animal?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A coelom is a body cavity found between the body wall and the gut. In a true coelom, this cavity is completely lined by mesoderm, the middle germ layer.
Coelomates are animals that have a true coelom lined by mesoderm. Examples given in NCERT are annelids, molluscs, arthropods, echinoderms, hemichordates and chordates.
In a true coelom the mesoderm fully lines the cavity on all sides. In a pseudocoelom the mesoderm is present only as scattered pouches between the ectoderm and endoderm, so the cavity is not fully lined. Aschelminthes are pseudocoelomates.
Acoelomate animals have no body cavity between the body wall and the gut. Platyhelminthes (flatworms) are the NCERT example of acoelomates.
Because the coelom type is a fixed feature used to classify animals into phyla, and NEET repeatedly tests which phylum is coelomate, pseudocoelomate or acoelomate. Getting the mesoderm-lining rule right lets you answer these questions quickly.
