Read Body Plans from Cells to Coelom
Animal classification uses organisation, symmetry, germ layers, body cavity and segmentation as increasingly specific clues.
Living-system questions are solved by matching a structure or organism to its function, process and stated condition.
Start with organisation and symmetry
Animals range from cellular organisation in sponges to organ-system organisation in more complex groups. Bilateral symmetry has one plane of symmetry; radial forms can be divided through multiple planes. Cnidarians and ctenophores are radial; adult echinoderms are radial though larvae are bilateral.
Read the body cavity
Acoelomates have no cavity between body wall and gut (flatworms); pseudocoelomates retain a cavity not fully lined by mesoderm (roundworms); true coelomates have a mesoderm-lined cavity (annelids and many later groups). Annelids are metamerically segmented and have closed circulation. Fasciola is acoelomate.
Add the free-living flatworm clue
Planaria is a free-living platyhelminth: it is triploblastic and acoelomate, and its ciliated epidermis supports gliding locomotion. Taenia and Fasciola are also acoelomate flatworms, but their parasitic tegument is not a ciliated locomotory surface. Obelia is diploblastic. When a question combines all three clues, select Planaria.
OlympIQ exam lens
- ✓Roundworm vs flatworm: pseudocoelom vs acoelom.
- ✓True coelom: mesoderm-lined cavity.
- ✓Adult echinoderm radial; larva bilateral.
- ✓Organs working as systems = organ-system level.
- ✓Planaria: triploblastic, acoelomate and ciliated.
Do not call every cavity a true coelom; check whether mesoderm fully lines it.
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