Q.How useful is the study of the nature of body cavity and coelom in the classification of animals?

The nature of the body cavity, or coelom, is one of the most useful characters for classification because it cleanly separates the triploblastic animals into three grades:

• Coelomates have a true body cavity lined by mesoderm — for example annelids, molluscs, arthropods, echinoderms, hemichordates and chordates.
• Pseudocoelomates have a cavity that is not lined by mesoderm; the mesoderm lies only as scattered pouches between the ectoderm and endoderm — for example the aschelminthes.
• Acoelomates have no body cavity at all between the gut and the body wall — for example the platyhelminthes.

Because the coelom depends on the mesoderm and reflects how complex the body plan is, examining it lets us assign an animal to the correct grade and helps mark the boundaries between phyla.

The coelom is the body cavity lying between the body wall and the gut wall. What matters for classification is not just whether this cavity is present, but whether it is lined by mesoderm, the third germ layer. Because the condition of the coelom is tied so closely to the triploblastic organisation, examining it gives a reliable way to sort animals.

On this basis, three conditions are distinguished:

• Coelomates. A true body cavity is present and is lined by mesoderm — this is the coelom in the strict sense. Annelids, molluscs, arthropods, echinoderms, hemichordates and chordates are all coelomates.
• Pseudocoelomates. A body cavity is present, but it is not lined by mesoderm; instead the mesoderm occurs only as scattered pouches between the ectoderm and endoderm. Such a partly bounded cavity is a pseudocoelom, seen in the aschelminthes.
• Acoelomates. There is no body cavity at all between the body wall and the gut. The platyhelminthes are the example.

The usefulness of the character is that these three grades broadly track the increasing complexity of the body plan and help mark the boundaries between phyla. Used together with the other fundamental features, the coelom lets a biologist confidently assign an animal to its correct group.

Method: The cross-section diagram approach

Rather than describing the three coelom conditions in words, mentally slice straight through the body — from the outer skin to the gut wall — for a series of phyla, and compare the resulting cross-sections directly.

• Cut through a flatworm (Platyhelminthes): the entire space between skin and gut is packed solid with tissue. There is no gap, no cavity at all — this is the acoelomate cross-section.
• Cut through a roundworm (Aschelminthes): an open space IS present, but its lining is incomplete — mesoderm sits only as scattered pouches, not as a continuous wall on both sides. This is the pseudocoelomate cross-section.
• Cut through an earthworm, or a vertebrate: there is a clear open cavity, and it is lined all the way around by a continuous mesodermal wall separating the gut from the body wall on every side. This is the true coelomate cross-section.

Lining up these three drawings side by side shows directly why the character is useful: it is a purely visual, unambiguous test — solid/no gap, gap with partial lining, or gap with full lining — that sorts every triploblastic phylum into exactly one of three buckets, without needing any other information about the animal's size, habitat or outward appearance. Because the sorting is structural rather than descriptive, it stays consistent across very different-looking members of the same phylum.

Conclusion: A side-by-side cross-section comparison shows, by what does or doesn't fill the body cavity, exactly why coelom type cleanly separates acoelomates, pseudocoelomates and coelomates — arriving at the same three-way classification as the descriptive definitions, but by direct visual comparison instead.