Which animal does not have red blood cells (RBCs)?

In most vertebrates, including humans, red blood cells (RBCs) play a crucial role in transporting oxygen throughout the body. RBCs contain hemoglobin, a protein that binds oxygen in the lungs (or gills) and delivers it to tissues. However, not all animals have red blood cells.

The correct answer is Earthworm (Option B). Unlike frogs, snakes, or crocodiles, earthworms belong to a group of invertebrates known as annelids. Their circulatory system is closed, meaning blood is confined to vessels, but it differs from vertebrates in a key way: they lack cellular red blood cells. Instead, earthworms have hemoglobin molecules freely dissolved in their blood plasma. This hemoglobin gives their blood a red color, but since it is not enclosed within cells, earthworms do not possess distinct RBCs.

Let’s compare with the other options:

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  Frog (A): Amphibians like frogs have typical vertebrate blood, including nucleated red blood cells.

  
  


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  Snake (C): Reptiles such as snakes also have RBCs, which are oval and nucleated.

  
  


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  Crocodile (D): Another reptile, crocodiles have red blood cells similar to other vertebrates.

  
  


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  Earthworm (B): No RBCs, but oxygen transport occurs via dissolved hemoglobin in plasma.

  
  

The absence of red blood cells in earthworms highlights the diversity of evolutionary adaptations in circulation. In invertebrates, oxygen transport varies widely:

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  Some rely on hemocyanin (copper-based pigments) instead of hemoglobin.

  
  


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  Others, like earthworms, depend on dissolved hemoglobin.

  
  


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  Some small invertebrates even rely on direct diffusion of oxygen through their skin.

  
  

This system works efficiently for earthworms because they are relatively small, have a moist skin surface for gas exchange, and do not require the higher oxygen transport efficiency that vertebrates need.

In conclusion, the correct answer is Earthworm, which transports oxygen without using red blood cells, relying instead on hemoglobin dissolved in its blood plasma.