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

Which of the following combinations of anatomical features uniquely distinguishes members of the phylum Annelida from members of the phylum Arthropoda?

  1. A closed circulatory system and metameric excretory nephridiaAnswer
  2. B
    Chitinous jointed appendages and a ventral nerve cord
  3. C
    An open circulatory system and excretion via Malpighian tubules
  4. D
    A rigid chitinous exoskeleton requiring periodic ecdysis

Answer

A closed circulatory system and metameric excretory nephridia
The correct answer highlights that annelids possess a closed circulatory system (where blood circulates strictly inside vessels) along with metameric nephridia present in each body segment for excretion. In contrast, arthropods feature an open circulatory system with a hemocoel and different excretory organs such as Malpighian tubules or green glands.

Step-by-Step Solution

1
Analyze the circulatory system differences between Annelida and Arthropoda
Annelids (e.g., earthworms) have a closed circulatory system where blood remains within blood vessels, whereas arthropods have an open circulatory system where hemolymph bathes organs in a hemocoel.
Circulatory configuration is a fundamental evolutionary distinction between these two coelomate phyla.
2
Compare the excretory structures of Annelida and Arthropoda
Annelids excrete waste using metamerically arranged nephridia in each segment, while arthropods use structures such as Malpighian tubules (insects/myriapods) or green glands (crustaceans).
Excretory organ mapping helps establish key phylum-level diagnostic features.
3
Evaluate the options to identify the unique combination for Annelida
The combination of a closed circulatory system and segmental nephridia belongs exclusively to Annelida when compared to Arthropoda.
Synthesizing anatomical and physiological criteria isolates the correct phylum characteristics.

Key Concept

Phylum Diagnostic Features of Annelida vs Arthropoda
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Topics

Higher Invertebrates: Annelida, Mollusca, Arthropoda, and Echinodermata

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