Annelids/Roundworms are the first animals to have blood vascular system.

Explanation

To determine whether Annelids or Roundworms are the first animals to have a blood vascular system, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Blood Vascular System**:
- A blood vascular system consists of a network of vessels (arteries and veins) that transport blood throughout the body, delivering oxygen and nutrients to tissues and removing waste products.

2. **Identifying Annelids**:
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