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Difficulty: MediumLower Invertebrates: Porifera, Coelenterata, Platyhelminthes, and Nematoda

Which of the following structural features distinguishes members of the phylum Platyhelminthes from Coelenterata?

  1. Possession of three distinct germ layers (triploblastic body plan)Answer
  2. B
    Possession of specialized stinging cells known as cnidocytes
  3. C
    Presence of a fluid-filled pseudocoelom between the gut and body wall
  4. D
    Presence of a complete digestive tract with separate mouth and anus

Answer

Possession of three distinct germ layers (triploblastic body plan)
Platyhelminthes (flatworms) represent the simplest animal group to exhibit a triploblastic body plan containing ectoderm, mesoderm, and endoderm. In contrast, Coelenterata (cnidarians) are diploblastic, having only an outer ectoderm and inner endoderm separated by non-cellular mesoglea.

Step-by-Step Solution

1
Analyze the germ layer organization of Coelenterata and Platyhelminthes.
Coelenterata are diploblastic (two germ layers: ectoderm and endoderm), while Platyhelminthes are triploblastic (three germ layers: ectoderm, mesoderm, and endoderm).
The evolutionary emergence of mesoderm in Platyhelminthes provides true muscle tissue and organs, distinguishing them from diploblastic coelenterates.
2
Evaluate the other body features to eliminate incorrect options.
Cnidocytes belong uniquely to coelenterates; pseudocoelom and complete digestive tract belong to roundworms (Nematoda).
Platyhelminthes are acoelomate and possess an incomplete digestive tract with a single opening.

Key Concept

Triploblastic organization in Platyhelminthes vs. diploblastic organization in Coelenterata
Estimated Time:1m 0s
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Topics

Lower Invertebrates: Porifera, Coelenterata, Platyhelminthes, and Nematoda