The structure that hydra uses to immobilize its prey is called

Step 1: Analyze the question The question asks about the structure Hydra uses to paralyze or immobilize its prey. Hydra is a freshwater cnidarian (coelenterate).

Step 2: Explain the concept Cnidarians (Hydra, jellyfish, sea anemones) have special stinging cells on their tentacles called cnidoblasts (or cnidocytes). Inside each cell is a capsule called a nematocyst (stinging capsule). It contains a coiled, thread-like tube, sometimes with toxin. When a prey touches the trigger (cnidocil), the thread shoots out, pierces or entangles the prey, and injects poison that paralyzes it. The tentacles then pass the prey to the mouth.

Step 3: Check each option

• A. Enteron: This is the digestive cavity (gut) of Hydra, not for capturing prey.
• B. Osteoblasts: Bone-forming cells in vertebrates.
• C. Osteocytes (written "osteocysts" here): Mature bone cells, which have nothing to do with Hydra.
• D. Nematocysts: Stinging capsules used to capture and immobilize prey. ✔
• E. Pneumatophores: Breathing roots of mangrove plants, which are plant structures.

Step 4: Answer The correct answer is D. Nematocysts.

Tip: Link the word parts: nemato = thread, cyst = capsule, so "thread capsule" is the stinging thread. Whenever you see Hydra, jellyfish, or sea anemone with "stinging" or "paralyze prey," think nematocyst/cnidoblast.