Aristotle divided animals into two groups based on the presence and absence of red blood cells.

Aristotle's Classification System:
Aristotle was a Greek philosopher who lived in the 4th century BCE. He was the first person to classify living organisms into groups based on their characteristics. He divided animals into two groups: those with red blood and those without. This classification was based on the observation that some animals had blood that was red in color, while others did not.

Animals with red blood:
Aristotle classified animals with red blood as "hot-blooded" or "warm-blooded" animals. This group included mammals, birds, and some fish. He believed that these animals had a special "vital heat" that kept them warm and active. He also believed that these animals had a soul that was different from other animals.

Animals without red blood:
Aristotle classified animals without red blood as "cold-blooded" or "cool-blooded" animals. This group included reptiles, amphibians, and some fish. He believed that these animals were less active and less intelligent than warm-blooded animals. He also believed that they had a different type of soul than warm-blooded animals.

Conclusion:
Aristotle's classification system was based on his observations and beliefs about the natural world. While his classification system is no longer used by modern scientists, it was an important step in the development of the scientific method. Today, scientists use a more complex classification system based on genetic and evolutionary relationships between organisms.