Asterias (starfish), Echinus (sea urchin) and Antedon (sea lily) are all examples of marine animals belonging to which phylum?

Introduction / Context:
Marine zoology introduces several unique phyla, one of which is Echinodermata. Members of this phylum, such as starfish and sea urchins, have distinctive features like spiny skin and a water vascular system. This question presents three classic examples and asks you to identify their phylum, testing your ability to link scientific names with their higher classification.

Given Data / Assumptions:

• The organisms listed are Asterias (starfish), Echinus (sea urchin) and Antedon (sea lily).
• All are marine animals commonly discussed under a single phylum.
• Options include Echinodermata, Annelida, Platyhelminthes and Arthropoda.
• We assume standard zoological classification terminology.

Concept / Approach:
Echinodermata is a phylum of exclusively marine animals characterized by radial symmetry in adults, a calcareous endoskeleton with spines, and a water vascular system with tube feet. Asterias (starfish), Echinus (sea urchin) and Antedon (sea lily) are standard textbook examples of echinoderms. Annelida includes segmented worms such as earthworms and leeches. Platyhelminthes includes flatworms. Arthropoda includes insects, crustaceans and arachnids with jointed legs and exoskeleton. None of these other phyla fit the structure of starfish or sea urchins.

Step-by-Step Solution:
Step 1: Recognize that Asterias, Echinus and Antedon are all marine animals with spiny bodies and often radial symmetry.Step 2: Recall that such animals are grouped under Echinodermata, which literally means spiny skinned animals.Step 3: Observe that Annelida contains segmented worms with elongated, bilateral bodies rather than radial, spiny bodies.Step 4: Note that Platyhelminthes includes flat, soft bodied worms, again very different from starfish and sea urchins.Step 5: Recognize that Arthropoda includes animals with jointed appendages and an exoskeleton, not tube feet and water vascular system.Step 6: Conclude that the three given examples belong to phylum Echinodermata.

Verification / Alternative check:
Standard zoology charts group starfish, sea urchins and sea lilies together under Echinodermata and mention their water vascular system, tube feet and calcareous endoskeleton. Diagrams show starfish as five armed animals, sea urchins as globular spiny spheres and sea lilies as stalked forms, all fitting the echinoderm body plan. None of these characteristics match the body structures of annelids, flatworms or arthropods, confirming that Echinodermata is the correct phylum.

Why Other Options Are Wrong:
Option B is wrong because Annelida includes earthworms and leeches, which lack radial symmetry and spiny skin. Option C is wrong because Platyhelminthes contains flatworms, which are soft bodied and usually parasitic or free living, not spiny marine animals. Option D is wrong because Arthropoda includes insects, crustaceans and arachnids, which have jointed legs and segmented exoskeletons, again unlike echinoderms.

Common Pitfalls:
Students sometimes confuse Echinodermata with Arthropoda because both groups may have spines or hard coverings. However, arthropods have an external exoskeleton and jointed limbs, whereas echinoderms have an internal calcareous skeleton and tube feet. Another pitfall is to forget that echinoderms are exclusively marine. Using the word root echino, meaning spiny, can help you remember that starfish and sea urchins are echinoderms. This association makes it easier to identify Echinodermata as the correct phylum in questions like this.

Final Answer:
Echinodermata, marine animals with spiny skin and water vascular system.