Blood does not clot while leach is sucking blood.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks why blood does not clot when a leech is sucking blood. This involves understanding the process of blood clotting and the role of leeches in this process.

2. **Identifying the Leech**: Leeches are classified as sanguivorous animals, meaning they feed exclusively on blood. This characteristic is crucial to understanding their feeding mechanism.

3. **Role of Saliva**: When a leech attaches to its host and begins to feed, it secretes saliva that contains specific enzymes. These enzymes play a critical role in preventing blood clotting.

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