What are protostomes?

Explanation

### Step-by-Step Solution:

1. **Definition of Protostomes**:
- Protostomes are a group of multicellular organisms characterized by their embryonic development. In these organisms, the first opening that forms during embryonic development, known as the blastopore, develops into the mouth.

2. **Embryonic Development**:
- During the early stages of development, the embryo forms a structure called the blastopore. In protostomes, this blastopore will eventually become the mouth of the organism.

...