Nereis, Pheretima (earthworm) and Hirudinaria (blood sucking leech) are examples of animals belonging to which phylum in the animal kingdom?

Introduction / Context:
This zoology question asks you to classify three well known worm like animals into the correct phylum. Nereis, Pheretima (earthworm), and Hirudinaria (leech) all share common structural features such as segmentation, which place them into a particular group in the animal kingdom. Recognising the phylum Annelida from such examples is an essential skill in animal diversity and classification topics.

Given Data / Assumptions:
- Three example animals are listed: Nereis, Pheretima, and Hirudinaria. - The options give four major invertebrate phyla. - The task is to identify the phylum common to all three animals. - Standard features of Coelenterata, Aschelminthes, Annelida, and Arthropoda are assumed.

Concept / Approach:
Phylum Annelida includes segmented worms with a true coelom and organ system level organisation. The body is divided into ring like segments, and they show features such as closed circulatory system in many members. Nereis is a marine polychaete worm, Pheretima is a terrestrial earthworm, and Hirudinaria is a parasitic leech. All three have segmented bodies and belong to Annelida. Coelenterata contains radially symmetrical aquatic animals like hydra and jellyfish. Aschelminthes includes roundworms with unsegmented bodies and pseudocoelom. Arthropoda contains joint legged animals such as insects, crustaceans, and spiders. Therefore, the correct phylum is Annelida.

Step-by-Step Solution:
Step 1: Recall that Nereis is a segmented marine worm often studied as a typical annelid. Step 2: Remember that Pheretima, the common earthworm, has a long cylindrical body with obvious metameric segmentation. Step 3: Recognise that Hirudinaria, the leech, also shows segmentation and belongs to the same phylum, though it is adapted for parasitic life. Step 4: Identify that segmentation and presence of a true coelom are key features of Annelida. Step 5: Compare with Coelenterata, which includes non segmented, radially symmetrical organisms like sea anemones and jellyfish. Step 6: Compare with Aschelminthes, where animals are round, unsegmented worms, not segmented annelids. Step 7: Compare with Arthropoda, whose members have jointed appendages and an exoskeleton, unlike worms and leeches. Step 8: Conclude that all three animals are members of phylum Annelida.

Verification / Alternative check:
In most zoology textbooks, examples for Annelida include Nereis (polychaete annelid), earthworm (Pheretima), and leech (Hirudinaria). These are often presented together to illustrate the diversity within the phylum, from marine to terrestrial to parasitic forms. Coelenterates like hydra and corals, aschelminths like Ascaris, and arthropods like cockroach appear in separate sections. The consistent grouping of Nereis, Pheretima, and Hirudinaria under Annelida confirms that this is the correct answer.

Why Other Options Are Wrong:
Coelenterata: This phylum includes animals such as hydra and jellyfish that show radial symmetry and a sac like body plan, not segmented worms. Aschelminthes: Members like Ascaris and Wuchereria are round, unsegmented worms and do not show the metameric segmentation of annelids. Arthropoda: Arthropods have jointed legs, a segmented body with exoskeleton, and include insects and crustaceans, which are different from soft bodied annelid worms.

Common Pitfalls:
Sometimes learners lump all worm like animals together and confuse annelids with roundworms or flatworms. The key difference is segmentation: annelids have repeated segments along the body, while roundworms and flatworms do not. Another pitfall is memorising examples without linking them to defining features. To avoid errors, always associate Nereis, earthworm, and leech with segmented body plan and phylum Annelida.

Final Answer:
Nereis, Pheretima, and Hirudinaria all belong to phylum Annelida.