Hydrostatic skeleton is found in

Options

A. Earthworm
B. Pigeon
C. Shark
D. Whale.

Answer

**Step-by-Step Solution:**

1. **Understanding Hydrostatic Skeleton:**
- A hydrostatic skeleton is a structure found in some animals, particularly invertebrates, where the body shape is maintained by the pressure of fluid within a body cavity.

2. **Identifying the Organisms:**
- The question provides four options: earthworm, pigeon, shark, and whale. We need to determine which of these organisms possesses a hydrostatic skeleton.

3. **Analyzing Each Option:**
- **Earthworm:** Earthworms are segmented invertebrates. Their body segments are filled with fluid and surrounded by muscles, allowing them to maintain shape and flexibility. This is a classic example of a hydrostatic skeleton.
- **Pigeon:** Pigeons are birds that possess a skeletal system made of bones, specifically pneumatic bones, which are not hydrostatic.
- **Shark:** Sharks have a cartilaginous skeleton, which is not classified as a hydrostatic skeleton.
- **Whale:** Whales, being mammals, have a bony skeleton, which also does not fit the definition of a hydrostatic skeleton.

4. **Conclusion:**
- Based on the analysis, the only organism among the options that has a hydrostatic skeleton is the earthworm.

**Final Answer:**
- The hydrostatic skeleton is found in **earthworm** (Option A).

---