All of the following are protostomes except :

Options

A. arthropods
B. annelids
C. echinoderms
D. molluscs

Explanation

To solve the question "All of the following are protostomes except:", we will analyze each option provided and determine which one does not belong to the protostome group.

### Step-by-Step Solution:

1. **Understand the Definition of Protostomes**:
- Protostomes are animals in which the blastopore (the first opening that forms during embryonic development) develops into the mouth.

2. **Identify the Characteristics of Protostomes**:
- In protostomes, the mouth is formed first.
- They exhibit determinant cleavage (the fate of each cell is determined early in development).
- They typically have spiral cleavage and a solid ventral nerve cord.

3. **Analyze the Options**:
- **Arthropods**: These are protostomes because their mouth develops from the blastopore.
- **Mollusks**: These are also protostomes for the same reason; the mouth forms first.
- **Annelids**: These too are protostomes, as they follow the same developmental pattern.
- **Echinoderms**: These are not protostomes. Instead, they are classified as deuterostomes, meaning their blastopore develops into the anus first, and the mouth forms later.

4. **Conclusion**:
- Since arthropods, mollusks, and annelids are all protostomes, the only option that does not belong to this group is **echinoderms**.

### Final Answer:
Echinoderms are the only group listed that is not a protostome.

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