Protostomic plan is observed in annelids, arthropods and molluscs.

Answer

### Step-by-Step Solution:

1. **Understanding Protostomic Plan**:
- The protostomic plan refers to a developmental pattern in certain animals where the first opening that forms during embryonic development becomes the mouth. This is a key characteristic of protostomes.

2. **Formation of Blastula**:
- During early embryonic development, a stage called the blastula is formed. This stage typically consists of 15 to 30 cells and contains a central cavity known as the blastocoel.

3. **Transition to Gastrula**:
- The blastula undergoes further development to form the gastrula. In this stage, the blastocoel begins to degrade, and cells start to aggregate, leading to the formation of different structures.

4. **Mouth and Anus Formation**:
- In protostomes, the aggregated cells that form the mouth develop from the blastopore, which is the first opening created during gastrulation. In contrast, in deuterostomes, the first opening becomes the anus.

5. **Examples of Protostomes**:
- Annelids (segmented worms), arthropods (insects, crustaceans), and molluscs (snails, clams) are all examples of protostomic organisms. They exhibit the characteristic of mouth formation from the blastopore.

6. **Conclusion**:
- Therefore, the statement that the protostomic plan is observed in annelids, arthropods, and molluscs is true, as these groups share the developmental feature of mouth formation from the blastopore during the gastrula stage.