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

A marine biologist dissects an adult invertebrate displaying pentamerous radial symmetry, an endoskeleton of calcareous ossicles, enterocoelous coelom development, and a hydraulic water vascular system. To which phylum does this organism belong, and through what mechanism is nitrogenous waste primarily excreted?

  1. Phylum Echinodermata; nitrogenous waste is excreted primarily by simple diffusion across the thin walls of tube feet and dermal branchiae.Answer
  2. B
    Phylum Annelida; nitrogenous waste is excreted primarily through metamerically arranged nephridia in each segment.
  3. C
    Phylum Arthropoda; nitrogenous waste is excreted primarily through Malpighian tubules discharging into the hindgut.
  4. D
    Phylum Mollusca; nitrogenous waste is excreted primarily through paired metanephridia (bojanus organs) opening into the mantle cavity.

Answer

Phylum Echinodermata; nitrogenous waste is excreted primarily by simple diffusion across the thin walls of tube feet and dermal branchiae.
The combination of pentamerous radial symmetry in adults, an endoskeleton composed of calcareous ossicles, enterocoelous coelom formation, and a hydraulic water vascular system specifically defines Phylum Echinodermata (such as sea stars and sea urchins). Unlike annelids, arthropods, and molluscs, echinoderms possess no specialized excretory organs; nitrogenous waste (chiefly ammonia) diffuses directly into the surrounding water via thin-walled tube feet and dermal branchiae (papulae).

Step-by-Step Solution

1
Analyze diagnostic anatomical traits given in the stem
Identified secondary pentamerous radial symmetry, calcareous ossicles, enterocoely, and a water vascular system as unique diagnostic features of adult echinoderms.
Echinoderms are the only higher invertebrate phylum possessing a water vascular system and enterocoelous development paired with radial symmetry in adults.
2
Determine the physiological excretory mechanism for the identified phylum
Echinoderms lack specialized excretory organs such as nephridia or Malpighian tubules.
Nitrogenous wastes (mostly ammonia) easily diffuse outward across high-surface-area thin membranes, namely tube feet and dermal branchiae (papulae).
3
Evaluate distractors based on cross-phylum organ misattributions
Confirmed that nephridia belong to Annelida, Malpighian tubules to terrestrial Arthropoda, and organs of Bojanus to Mollusca.
Matching the correct phylum and its corresponding excretory physiology eliminates all distractor choices.

Key Concept

Diagnostic anatomical features and excretory mechanisms of Echinodermata compared to other higher invertebrate phyla
Estimated Time:1m 40s
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Topics

Higher Invertebrates: Annelida, Mollusca, Arthropoda, and Echinodermata

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