Nematocysts occur in

Options

A. Ctenophora
B. Planaria
C. Fasciola
D. Obelia.

Answer

To solve the question "Nematocysts occur in," we will analyze the given options step by step:

### Step 1: Understand what Nematocysts are
- Nematocysts are specialized cells found in certain organisms that contain a stinging organelle. These are primarily associated with the phylum Cnidaria, which includes jellyfish, corals, and sea anemones.

**Hint:** Nematocysts are unique to a specific group of organisms known for their stinging ability.

### Step 2: Analyze the options provided
- **Option A: Tenophora** - This phylum includes organisms known as comb jellies. They do not possess nematocysts; instead, they have a different type of cell for capturing prey.

**Hint:** Consider whether the organisms in this option are known for having stinging cells.

- **Option B: Planaria** - Planaria are flatworms that belong to the phylum Platyhelminthes. They do not have nematocysts, as this feature is not characteristic of flatworms.

**Hint:** Think about the classification of Planaria and whether they belong to the same group as Cnidarians.

- **Option C: Fasciola** - Fasciola, or liver flukes, are also members of the phylum Platyhelminthes. Like Planaria, they do not possess nematocysts.

**Hint:** Similar to Planaria, consider the characteristics of Fasciola and their classification.

- **Option D: Obelia** - Obelia is a genus within the phylum Cnidaria. Members of this phylum are known for their nematocysts, which they use for defense and capturing prey.

**Hint:** Identify if this option is part of the group that is known for having stinging cells.

### Step 3: Conclusion
After analyzing all the options, we can conclude that nematocysts occur in **Obelia (Option D)**, as it is a member of the phylum Cnidaria, which is characterized by the presence of these stinging cells.

**Final Answer:** Nematocysts occur in **Obelia**.