Which of the following animals has an open circulatory vascular system in which blood flows through body cavities rather than exclusively through closed blood vessels?

Introduction / Context:
Animals have evolved different types of circulatory systems to transport nutrients, gases, and waste products throughout the body. In some groups such as vertebrates, blood circulates through a closed network of vessels. In others, blood or haemolymph flows more freely through body cavities in what is called an open circulatory system. This question asks you to identify which listed animal possesses an open circulatory system.

Given Data / Assumptions:

• Options include cockroach, human, rat, pigeon, and frog.
• Vertebrates such as humans, rats, pigeons, and frogs are known to have closed circulatory systems.
• Cockroach is an arthropod insect.
• We assume standard zoological descriptions of circulatory system types.

Concept / Approach:
In an open circulatory system, the circulating fluid called haemolymph is not always confined to blood vessels. Instead, it is pumped by a heart into body cavities called sinuses, where it directly bathes tissues and then returns to the heart. This type of system is found in many invertebrates, including most arthropods and molluscs. In a closed circulatory system, blood remains within vessels and capillaries. Vertebrates such as mammals, birds, and amphibians have closed systems. Cockroach, being an insect, has an open circulatory system.

Step-by-Step Solution:
Step 1: Identify cockroach as an insect and thus an arthropod. Step 2: Recall that arthropods typically have open circulatory systems with dorsal hearts pumping haemolymph into body sinuses. Step 3: Recognise that humans, rats, pigeons, and frogs are vertebrates with well developed hearts and networks of arteries, veins, and capillaries. Step 4: In vertebrates, blood stays inside vessels as it circulates, which defines a closed circulatory system. Step 5: Since the question asks for an animal with an open system, attention focuses on cockroach. Step 6: Confirm that cockroach has a tubular heart and haemocoel body cavity filled with haemolymph. Step 7: Conclude that cockroach is the correct answer.

Verification / Alternative check:
Standard zoology descriptions of cockroach anatomy state that it has a dorsal heart with multiple chambers which opens into the haemocoel. Haemolymph circulates through perivisceral and perineural sinuses rather than a closed vessel network. In contrast, diagrams of human, rat, pigeon, and frog circulation show hearts with atria and ventricles, major arteries like the aorta, veins, and dense networks of capillaries, illustrating a closed circulatory system. This consistent distinction supports cockroach as the animal with an open system in the given options.

Why Other Options Are Wrong:
Human beings have a four chambered heart and a closed network of blood vessels. Rat, as a mammal, also has a closed circulatory system similar to humans. Pigeon, a bird, has a four chambered heart and closed circulation adapted for high metabolic rates. Frog, an amphibian, has a three chambered heart, but its blood still circulates only within vessels, so it is also a closed system. None of these vertebrates exhibit an open circulatory pattern.

Common Pitfalls:
Learners sometimes think of open and closed systems only in terms of complexity and may assume that smaller animals have open systems and larger ones have closed systems. While this is sometimes true, the correct classification depends on the actual anatomy. Remember that most arthropods and many molluscs have open circulatory systems, while vertebrates have closed systems. Associating cockroach and other insects with open circulation helps answer such questions quickly.

Final Answer:
The animal with an open circulatory system is the Cockroach.