In vertebrates, the …………. Is/are usually replaced by …..

Options

A. dorsal hollow nerve cord, a ventral solid nerve cord
B. gill arches, pharyngeal pouches
C. notochord, a dorsal hollow nerve cord
D. notochord, a vertebral column

Explanation

**Step-by-Step Solution:**

1. **Understanding the Structures:**
- In vertebrates, there are several key structures that are important to understand, including the notochord and the vertebral column.

2. **Identifying the Notochord:**
- The notochord is a flexible rod-like structure made of a material similar to cartilage. It serves as a primary skeletal structure in the early development of vertebrates.

3. **Transition to Vertebral Column:**
- During the development of vertebrates, the notochord is typically replaced by the vertebral column (also known as the backbone). This process is crucial for the structural support and protection of the spinal cord.

4. **Conclusion:**
- Therefore, in vertebrates, the notochord is usually replaced by the vertebral column.

**Final Answer:**
In vertebrates, the notochord is usually replaced by the vertebral column.

---