Which of the following phyla are considered diploblastic (i.e. consisting of two
germ layers)?
Echinodermata
Chordata
Mollusca
Cnidaria
Among the given options, only Cnidaria is diploblastic; Echinodermata, Chordata and Mollusca are triploblastic animals with three germ layers (ectoderm, mesoderm, endoderm).
Introduction
In animal embryology, the number of germ layers defines whether an animal is diploblastic (two layers) or triploblastic (three layers). Among the phyla Echinodermata, Chordata, Mollusca and Cnidaria, only Cnidaria shows a diploblastic organisation, while the others are triploblastic and have an additional mesodermal layer.
What is diploblastic organization?
Diploblastic animals develop from two primary germ layers: an outer ectoderm and an inner endoderm (or gastrodermis), with a non‑cellular layer such as mesoglea between them. These animals lack mesoderm and therefore do not show complex organ systems like well‑developed muscles, circulatory systems or true coelom.
Option analysis: which phylum is diploblastic?
Cnidaria – Correct option
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Cnidarians (jellyfish, hydra, corals, sea anemones) have a body wall made of two germ layers: an outer epidermis derived from ectoderm and an inner gastrodermis derived from endoderm, separated by mesoglea.
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Because they possess only ectoderm and endoderm and completely lack mesoderm, Cnidaria is a classic diploblastic phylum and the correct answer in this question.
Echinodermata – Incorrect (triploblastic)
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Echinoderms (starfish, sea urchins, sea cucumbers) are deuterostomes with three germ layers: ectoderm, mesoderm and endoderm.
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Their well‑developed mesoderm forms a true coelom, endoskeleton and complex organ systems, which clearly places them among triploblastic animals rather than diploblasts.
Chordata – Incorrect (triploblastic)
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Chordates (including vertebrates) also develop three germ layers in the gastrula: ectoderm, mesoderm and endoderm.
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The mesoderm gives rise to structures such as notochord, musculature, skeletal elements and circulatory system, confirming that Chordata is triploblastic, not diploblastic.
Mollusca – Incorrect (triploblastic)
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Molluscs (snails, bivalves, squids, octopuses) are coelomate animals with ectoderm, mesoderm and endoderm, and are classified as triploblastic.
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Presence of mesoderm‑derived muscular foot, visceral mass and complex organ systems clearly indicates a triploblastic body plan, so Mollusca is not diploblastic.
Key comparison table (SEO‑friendly)
| Phylum | Germ layers | Diploblastic / Triploblastic | Key reason for classification |
|---|---|---|---|
| Cnidaria | Ectoderm + endoderm only | Diploblastic | Two germ layers with mesoglea, no mesoderm |
| Echinodermata | Ectoderm + mesoderm + endoderm | Triploblastic | Deuterostome coelomate with mesodermal endoskeleton |
| Chordata | Ectoderm + mesoderm + endoderm | Triploblastic | Mesoderm forms notochord, muscles, circulatory system |
| Mollusca | Ectoderm + mesoderm + endoderm | Triploblastic | Mesoderm‑derived muscular foot and organs |
For the given MCQ, the correct phylum that is diploblastic is Cnidaria.
