Biology · Animal Kingdom · NEET
| Body cavity | Acoelomate: absent (filled with tissue) | Coelomate: present, fully lined by mesoderm |
| Mesoderm lining | Pseudocoelomate: scattered, incomplete (body-wall side only) | Coelomate: complete, both gut and body-wall sides |
| NCERT example | Acoelomate: Platyhelminthes; Pseudocoelomate: Aschelminthes | Coelomate: Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata, Chordata |
Both are body cavities (a space between the gut and the body wall). The difference is the lining. A true coelom is fully lined by mesoderm on BOTH the outer body-wall side AND the inner gut side. A pseudocoelom has mesoderm only in scattered patches, usually towards the body wall, but NOT lining the gut. So a pseudocoelom is a cavity that is only partly lined by mesoderm. NEET often tests this exact line.
Platyhelminthes (flatworms) are ACOELOMATE. This is the most common trap in NEET. Flatworms have NO body cavity at all. The space between the gut and body wall is filled with tissue (mesenchyme/parenchyma), so there is no fluid-filled cavity. If a question says platyhelminthes are pseudocoelomate, that statement is WRONG.
Aschelminthes (roundworms, like Ascaris) are PSEUDOCOELOMATE. They DO have a cavity, but the mesoderm lining it is scattered and incomplete, not a full lining. NCERT says the mesoderm is present as scattered pouches between the ectoderm and endoderm. So they are not acoelomate (they have a cavity) and not true coelomate (the lining is incomplete).
NCERT lists six groups: Annelids, Molluscs, Arthropods, Echinoderms, Hemichordates and Chordates. A quick way to remember: everything from Annelida onward in the NCERT phylum order (except keep in mind molluscs are unsegmented). These animals have a body cavity completely lined by mesoderm.
Mesoderm is the middle germ layer. When we say a cavity is 'lined by mesoderm', it means a thin sheet of mesoderm forms the wall of the cavity, wrapping around both the gut and the inner body wall. This lining is called the peritoneum in true coelomates. In pseudocoelomates this lining is missing on the gut side, and in acoelomates there is no cavity to line.
A true coelom gives space for internal organs to grow, move and fold (like a long coiled gut). It acts like a cushion, protecting organs, and it can act as a hydrostatic skeleton for movement. This is why more advanced animals are coelomates. NEET may ask you to link 'coelom' with organ-system level of organisation.
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.
Bilaterally symmetrical and acoelomate animals are exemplified by
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 coelom of that animal?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A coelom is the fluid-filled body cavity found between the gut wall and the outer body wall, and it is lined by mesoderm.
Coelomate: Earthworm (Annelida). Pseudocoelomate: Ascaris/roundworm (Aschelminthes). Acoelomate: Planaria/tapeworm (Platyhelminthes).
No. All these three groups are triploblastic (have mesoderm), but only coelomates have a fully mesoderm-lined cavity. Platyhelminthes are triploblastic yet acoelomate.
Poriferans do not have a true coelom, so in NEET they are treated as acoelomate. They are also diploblastic, so the mesoderm-based coelom classification mainly starts from Platyhelminthes onward.
Yes, Mollusca is a true coelomate, but note it is unsegmented. NEET uses this to reject answer options like 'Annelida, Arthropoda and Mollusca' when segmentation is required.
