Amphioxus belongs to the phylum_____,.

Explanation

**Step-by-Step Solution:**

1. **Identify the Organism**: The question asks about Amphioxus, which is a marine organism.

2. **Understand the Classification**: Amphioxus is classified under a specific phylum. To determine this, we need to look at its characteristics.

3. **Characteristics of Amphioxus**: Amphioxus exhibits features that are intermediate between invertebrates and vertebrates. It has a notochord that extends from the head to the tail, which is a key characteristic of chordates.

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