Define a holobranch.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Term**: The term "holobranch" refers to a specific anatomical structure found in certain aquatic organisms, particularly fish.

2. **Definition**: A holobranch is defined as a complete gill. This means it encompasses all the necessary components that make up a functional gill.

3. **Structure**: Holobranchs consist of bony or cartilaginous arches. These structures provide support and are essential for the gill's functionality.

...