Identify the incorrect statement

The correct option is "occurs in all vertebrates".

To understand why this is the incorrect statement (and thus the correct answer to the question), let us break down the concepts of double circulation.
Double circulation is a process where blood passes through the heart twice during one complete cycle of the body. It consists of two primary pathways:

1. Pulmonary circulation: This loop carries deoxygenated blood away from the heart, to the lungs where it is oxygenated, and then returns oxygenated blood back to the heart.

2. Systemic circulation: This loop carries oxygenated blood away from the heart to the rest of the body, and returns deoxygenated blood back to the heart.

Now, let us analyze the distribution of double circulation across the animal kingdom to evaluate the options:

Double circulation is fully developed in birds and mammals, which have a complete 4-chambered heart (two atria and two ventricles). This prevents the mixing of oxygenated and deoxygenated blood, ensuring highly efficient oxygen delivery. Certain reptiles, like crocodilians, also exhibit double circulation.
However, double circulation does not occur in all vertebrates:

- Fish: Fish possess a 2-chambered heart (one atrium and one ventricle) and exhibit single circulation, where blood passes through the heart only once per cycle (Heart ⇒ Gills ⇒ Body tissues ⇒ Heart).

- Amphibians and most reptiles: These organisms possess a 3-chambered heart (two atria and one ventricle) and exhibit incomplete double circulation, where oxygenated and deoxygenated blood mix in the single ventricle.

Therefore, stating that double circulation "occurs in all vertebrates" is scientifically incorrect, making it the correct option for this question.