What is common between Hydra and Obelia ?

Options

A. Both show polymorphism
B. Both posses a larval stage in their life history
C. Both are found in fresh water
D. Both have a jelly like mesoglea without any cells

Answer

### Step-by-Step Solution:

1. **Identify the Phylum and Class**:
- Both Hydra and Obelia belong to the phylum **Cnidaria** and the class **Hydrozoa**. This classification indicates that they share certain fundamental biological characteristics.

2. **Examine Polymorphism**:
- Polymorphism refers to the occurrence of different forms or stages in the life cycle of an organism.
- Obelia exhibits two body forms: the **polyp** (a stationary form) and the **medusa** (a free-swimming form).
- Hydra, however, only has the **polyp** form and does not have a medusa stage. Therefore, while they both show polymorphism, it is more pronounced in Obelia.

3. **Larval Stages**:
- Both organisms have a larval stage in their life cycle.
- Hydra has a **planula larva**, while Obelia has an **amphiblastula larva**. This indicates that they both undergo a larval phase, but the types of larvae are different.

4. **Habitat**:
- Hydra is primarily found in **freshwater** environments, while Obelia is predominantly a **marine** organism. This means they do not share a common habitat.

5. **Mesoglea**:
- Both Hydra and Obelia possess a jelly-like substance called **mesoglea**.
- This mesoglea is acellular (lacking cells) and plays a crucial role in providing structural support and facilitating movement through hydrostatic pressure.

6. **Conclusion**:
- The common feature between Hydra and Obelia is the presence of **jelly-like mesoglea** without any cells. This characteristic is significant as it contributes to their structural integrity and function.

### Final Answer:
The common feature between Hydra and Obelia is that both have a **jelly-like mesoglea** without any cells.

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