Name an animal group with open circulatory system.

Explanation

**Step-by-Step Solution:**

1. **Understanding Open Circulatory System**: An open circulatory system is characterized by the absence of a closed network of blood vessels. Instead, blood (or hemolymph) flows freely within the body cavity, bathing the organs directly.

2. **Identifying Organisms with Open Circulatory System**: The primary group of animals that exhibit an open circulatory system is the arthropods. This includes insects, arachnids (like spiders), and crustaceans (like crabs and lobsters).

3. **Characteristics of Arthropods**: Arthropods are invertebrates, meaning they lack a backbone. They have a segmented body, an exoskeleton made of chitin, and jointed appendages. The simplicity of their circulatory system is suitable for their size and lifestyle.

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