Among the following animals, which one does not possess red blood cells, because its respiratory pigment is dissolved directly in the blood plasma instead of being carried by distinct red cells?

Introduction / Context:
In many animals, oxygen is transported by respiratory pigments such as hemoglobin that may be carried inside specialised cells called red blood cells. In some invertebrates, however, the respiratory pigment is dissolved directly in the blood plasma and there are no distinct red blood cells. This question tests knowledge of which listed animal lacks red blood cells and instead circulates dissolved pigment in its body fluid.

Given Data / Assumptions:

• The animals listed are frog, earthworm, snake, peacock, and an extra bird option.
• We must identify the animal without red blood cells.
• We assume understanding that vertebrates generally have red blood cells.
• We also assume familiarity with earthworm circulation as described in basic zoology.

Concept / Approach:
Vertebrates such as frogs, snakes, peacocks, and pigeons have red blood cells that carry hemoglobin. Earthworms, which are invertebrates belonging to phylum Annelida, have hemoglobin dissolved directly in their blood plasma. They do not have distinct red blood cells like vertebrates. Therefore, the correct approach is to identify earthworm as the animal where the respiratory pigment is present in the fluid but not within specialised red blood cells.

Step-by-Step Solution:
Step 1: Recognise that frog, snake, peacock, and pigeon are vertebrates and typically have red blood cells with hemoglobin. Step 2: Recall that earthworm is an invertebrate with a closed circulatory system but without discrete red blood cells. Step 3: Understand that in earthworms, hemoglobin is dissolved in the plasma of the blood, giving it a reddish colour. Step 4: Compare this property with the requirement in the question, which asks for an animal without red blood cells. Step 5: Select earthworm as the correct answer.

Verification / Alternative check:
To verify, we can recall diagrams of vertebrate blood showing red blood cells, white blood cells, and platelets. Textbooks describe such blood components for frogs, birds, and mammals. For earthworms, the description emphasises a closed circulatory system with blood vessels, but hemoglobin is reported to be in solution rather than contained within cells. This difference is often highlighted in exam oriented summaries of annelid circulation, reinforcing that earthworms lack red blood cells.

Why Other Options Are Wrong:
Option A: Frog is a vertebrate amphibian and has nucleated red blood cells that carry hemoglobin. Option C: Snake is a vertebrate reptile with red blood cells containing hemoglobin. Option D: Peacock is a bird and has red blood cells, though they are nucleated, like other non mammalian vertebrates. Option E: Pigeon is also a bird with nucleated red blood cells, so it does not fit the description of lacking red blood cells.

Common Pitfalls:
Students may mistakenly think that all animals with red coloured blood must have red blood cells, or may forget that some invertebrates have dissolved pigments. Another error is to assume that earthworms, because they appear simple, might lack hemoglobin altogether, which is not correct. Instead, they have hemoglobin dissolved in plasma. The key point to remember is that vertebrates have red blood cells, while certain invertebrates like earthworms carry hemoglobin in solution without specialised red blood cells.

Final Answer:
The animal among the options that does not possess red blood cells, because its respiratory pigment is dissolved directly in the plasma, is the earthworm.