In animal classification, earthworms belong to which phylum based on their elongated, segmented, and cylindrical body structure?

Introduction / Context:
Classification of animals into phyla is based on body structure, organization, and other features. Earthworms are common examples used in school biology to represent one particular phylum. This question checks whether you can recall the correct phylum name that matches the segmented body plan of earthworms.

Given Data / Assumptions:

• The organism in question is an earthworm.
• We must choose the correct phylum from a list.
• Basic knowledge of animal body plans is assumed.

Concept / Approach:
Earthworms have a long, cylindrical, and segmented body. Their segmentation is both external and internal, with repeated segments called metameres. They are triploblastic, coelomate animals with a true body cavity and closed circulatory system. These characteristics are typical of the phylum Annelida. The term annelid comes from a word meaning little ring, referring to the segmented appearance of the body.

Step-by-Step Solution:
Step 1: Identify that earthworms show true segmentation and a tube within a tube body plan. Step 2: Recall that such segmented worms, including leeches and marine worms, belong to phylum Annelida. Step 3: Compare this with features of other phyla like Protozoa, Cnidaria, and Mollusca, which do not fit the earthworm body type. Step 4: Select Annelida as the correct phylum for earthworms.

Verification / Alternative check:
Standard classification tables group organisms such as Nereis, earthworm, and leech together in phylum Annelida. They all share segmentation, a coelom, and a closed circulatory system. Since earthworms clearly belong to this group, Annelida is confirmed as the correct answer.

Why Other Options Are Wrong:

• Option A (Protozoa): Protozoans are single celled organisms like Amoeba and Paramecium, not multicellular segmented worms.
• Option B (Cnidaria): Cnidarians include jellyfish and corals, which have radial symmetry and tentacles, not a segmented worm like structure.
• Option D (Mollusca): Molluscs include snails, clams, and octopus, which have soft, unsegmented bodies often protected by shells.

Common Pitfalls:
A common error is to confuse Annelida with Arthropoda or Mollusca due to superficial similarities. Remember that true external segmentation with repeated rings and lack of jointed appendages is characteristic of annelids. Using model organisms like earthworms can help you remember these classification details more clearly.

Final Answer:
Earthworms belong to the phylum Annelida.