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Difficulty: HardHigher Invertebrates: Annelida, Mollusca, Arthropoda, and Echinodermata

In comparative invertebrate zoology, body symmetry, coelomic origin, and organ system organization are critical diagnostic criteria for phylum classification. Which of the following features uniquely distinguishes adult members of the phylum Echinodermata from adult members of Annelida, Mollusca, and Arthropoda?

  1. Secondary pentaradial symmetry combined with a water vascular system of enterocoelous originAnswer
  2. B
    Metameric body segmentation coupled with paired metanephridia in each coelomic compartment
  3. C
    A triploblastic body plan with three distinct tagmata and four pairs of jointed ambulatory appendages
  4. D
    A schizocoelous coelom reduced to a pericardial cavity and an open circulatory system forming a haemocoel

Answer

Secondary pentaradial symmetry combined with a water vascular system of enterocoelous origin is the unique diagnostic combination distinguishing adult echinoderms from annelids, molluscs, and arthropods.
Adult echinoderms (such as sea stars and sea urchins) undergo a radical metamorphosis from bilaterally symmetrical larvae to adult forms displaying secondary pentaradial symmetry. Furthermore, their coelom is enterocoelous in origin and gives rise to a specialized water vascular system utilized for locomotion, gas exchange, and feeding. This combination of traits is unique among higher invertebrates.

Step-by-Step Solution

1
Analyze the embryological lineage and symmetry of Echinodermata
Echinoderms belong to the deuterostome clade, forming their coelom via enterocoely. While their larvae are bilaterally symmetrical, adults develop secondary pentaradial symmetry.
Establishing symmetry and coelomic origin distinguishes deuterostomes from protostomes.
2
Identify unique organ system structures in Echinodermata
The water vascular system (hydrovascular system with tube feet) is found exclusively in echinoderms.
This system operates locomotion, food capture, and respiration, serving as a primary diagnostic feature of the phylum.
3
Compare against diagnostic features of Annelida, Mollusca, and Arthropoda
Annelids (metamerism, nephridia), Arthropods (jointed appendages, tagmata, chitinous exoskeleton), and Molluscs (mantle, radula, haemocoel) are all schizocoelous protostomes with bilateral symmetry.
Systematic elimination confirms that pentaradial symmetry and the enterocoelous water vascular system are exclusive to adult echinoderms.

Key Concept

Diagnostic features of Echinodermata compared to protostomic higher invertebrates
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Topics

Higher Invertebrates: Annelida, Mollusca, Arthropoda, and Echinodermata

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