Adamsia (sea anemone) and Pennatula (sea pen) are marine animals that are examples of members of which animal phylum?

Introduction / Context:
This zoology question focuses on the classification of simple aquatic animals into different phyla. Adamsia, commonly called sea anemone, and Pennatula, known as sea pen, are classic textbook examples used to illustrate a specific phylum. Correctly associating these genera with their phylum is important for building a solid foundation in invertebrate zoology and animal diversity.

Given Data / Assumptions:
- Two example animals are given: Adamsia (sea anemone) and Pennatula (sea pen). - The options list four animal phyla. - Only one phylum correctly includes both of these animals. - Standard school level zoological classification is assumed.

Concept / Approach:
Sea anemones and sea pens are soft-bodied, radially symmetrical marine animals with a central gastrovascular cavity and tentacles surrounding a mouth. They exhibit tissue level organisation and have a single opening functioning as both mouth and anus. These are characteristic features of phylum Coelenterata, also called Cnidaria. Members of this phylum typically possess stinging cells called cnidocytes. Aschelminthes are pseudocoelomate roundworms, Annelida contains segmented worms like earthworms and leeches, and Arthropoda includes joint legged animals such as insects and crustaceans. Therefore, Adamsia and Pennatula belong to Coelenterata.

Step-by-Step Solution:
Step 1: Recall that sea anemones and sea pens are marine animals with a sac like body and tentacles. Step 2: Recognise that these animals have radial symmetry and a single gastrovascular cavity, typical of coelenterates. Step 3: Remember that phylum Coelenterata, also known as Cnidaria, includes hydra, jellyfish, sea anemones, and corals. Step 4: Evaluate Aschelminthes, which are unsegmented roundworms living in various habitats and do not look like sea flowers or pens. Step 5: Evaluate Annelida, whose members such as earthworms and leeches are bilaterally symmetrical, segmented worms, not radially symmetrical animals like sea anemones. Step 6: Evaluate Arthropoda, which includes animals with jointed appendages and exoskeleton, a completely different body plan from Adamsia or Pennatula. Step 7: Conclude that the correct phylum for both Adamsia and Pennatula is Coelenterata.

Verification / Alternative check:
Textbooks commonly list examples for each phylum to make learning easier. Under Coelenterata, you will usually find hydra, Aurelia (jellyfish), Adamsia (sea anemone), and Pennatula (sea pen) highlighted. Diagrams show their polyp and colony forms, and descriptions emphasise cnidocytes and radial symmetry. No such association is made with Aschelminthes, Annelida, or Arthropoda. This repeated and consistent grouping in study material verifies that Coelenterata is the correct phylum.

Why Other Options Are Wrong:
Aschelminthes: This phylum consists of roundworms like Ascaris and Wuchereria that are bilaterally symmetrical, pseudocoelomate worms and do not resemble sea anemones or sea pens. Annelida: Members such as Nereis and earthworm are segmented worms with bilateral symmetry and closed circulatory systems, not radially symmetrical polyps. Arthropoda: This phylum includes insects, spiders, and crustaceans, all characterised by jointed legs and exoskeleton, which are absent in Adamsia and Pennatula.

Common Pitfalls:
Students sometimes confuse Coelenterata with Annelida because both names start with a similar sound and both include some aquatic species. Another pitfall is mixing up phylum names when several marine animals are studied at the same time. To avoid these mistakes, remember the specific traits of coelenterates such as radial symmetry, cnidocytes, and body forms like polyps and medusae. Associating sea anemone and sea pen clearly with Coelenterata helps fix the classification in memory.

Final Answer:
Adamsia and Pennatula are examples of animals belonging to the phylum Coelenterata.