In which of the following phylum, would you place an animal with a bilaterally symmetrical, triploblastic body without a true internal cavity (coelom)?

Correct — B, Platyhelminthes. The three features in the stem pick out one phylum between them. Flatworms are the first group in the animal series to show bilateral symmetry together with three germ layers, and they have no body cavity between the gut and the body wall — the space is packed with mesodermal tissue. An animal that is bilateral, triploblastic and acoelomate is a flatworm.

• (a)Porifera — Sponges are the simplest animals of all — mostly asymmetrical, without true tissues, and not triploblastic. They fail the stem on almost every count.
• (c)Cnidaria — Cnidarians such as Hydra and jellyfish are radially symmetrical and diploblastic, with only two germ layers and a jelly-like mesogloea between them.
• (d)Annelida — Annelids such as the earthworm are bilateral and triploblastic, but they possess a true coelom lined by mesoderm. They are the standard example of the coelomate condition, which is what the stem excludes.

Animal phyla are sorted by a short sequence of body-plan decisions: whether there is symmetry and of what kind, how many germ layers form in the embryo, and whether a body cavity develops between the gut and the body wall. Acoelomate animals have no such cavity, pseudocoelomates have one that is not fully lined by mesoderm, and coelomates have a true cavity lined by mesoderm on both sides.

The four options are laid out along that very sequence, which is why the item is quick once the sequence is held in order. Porifera fails at symmetry and tissues, Cnidaria fails at germ layers, Annelida passes both but fails at the cavity, and Platyhelminthes is what remains. Flatworms include free-living planarians as well as the parasitic flukes and tapeworms; the absence of a coelom is one reason they have no circulatory system and must stay thin enough for material to diffuse through the body.

• Platyhelminthes are bilaterally symmetrical, triploblastic and acoelomate.
• Porifera are the sponges, cellular in grade of organisation and without true tissues.
• Cnidaria are radially symmetrical and diploblastic, with a mesogloea between the two layers.
• Annelida are triploblastic coelomates with a true, mesoderm-lined body cavity and a segmented body.
• Nematodes occupy the middle case, having a pseudocoelom that is not fully lined by mesoderm.

• Confusing the pseudocoelom of roundworms with the complete absence of a cavity in flatworms.
• Reading 'bilateral' as sufficient on its own, when annelids are bilateral too.
• Calling cnidarians triploblastic because of the mesogloea, which is not a true germ layer.

A features-to-phylum item. Each printed characteristic knocks out one option, so the phyla are usually chosen to be eliminated in sequence.

• practice — not a real PYQ

  Which one of the following animals possesses a true coelom?

  • (a)Planaria
  • (b)Hydra
  • (c)Earthworm
  • (d)Sponge

  Answer(c) Earthworm — annelids have a body cavity fully lined by mesoderm, which flatworms and cnidarians lack.

• practice — not a real PYQ

  Diploblastic organisation, with only two germ layers, is characteristic of

  • (a)Platyhelminthes
  • (b)Cnidaria
  • (c)Annelida
  • (d)Arthropoda

  Answer(b) Cnidaria — an ectoderm and an endoderm with a non-cellular mesogloea between them.