Diploblastic animals have two primary germ layers, ectoderm and endoderm. Cnidarians have mesoglea between them; mesoglea is not a third germ layer equivalent to mesoderm. Triploblastic animals develop mesoderm between the other two layers. Flatworms and the subsequent bilaterian groups discussed here are triploblastic.

A coelom is a body cavity between the body wall and gut that is lined by mesoderm. In acoelomates such as flatworms, no such cavity separates the wall from the gut. Roundworms are pseudocoelomates: their cavity is not completely lined by mesoderm. Annelids, molluscs, arthropods, echinoderms, hemichordates and chordates belong to coelomate groups, though the adult extent and modification of the cavity vary.

The gut lumen is not the coelom. A diagram can show a space inside the gut even in an acoelomate. Also, being triploblastic does not establish a true coelom: both flatworms and roundworms are counterexamples. To interpret a cross-section, locate the gut wall first and then inspect the tissue lining the separate cavity outside it.

Worked example: A three-layered animal with a cavity lacking a complete mesodermal lining is pseudocoelomate, not diploblastic and not necessarily a true coelomate.

NCERT trap: Do not count mesoglea as mesoderm or mistake gut lumen for body cavity.

NEET method: Use two independent questions: how many germ layers, and how is the space outside the gut lined?

Reading estimate; allow another 3–5 minutes for the checks.

Animal Kingdom

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