Q.Differentiate between an open and a closed circulatory system, naming one phylum studied in this chapter that shows each.

Step 1. In a closed circulatory system, blood remains confined to vessels throughout its circuit and never directly bathes the tissues; Annelida shows this system.

Step 2. In an open circulatory system, the heart pumps blood (called haemolymph, since it is not vessel-confined) into open body sinuses collectively called the haemocoel, through which it directly bathes the tissues before draining back to the heart; Arthropoda shows this system (Mollusca too, except in cephalopods).

Step 3. A closed system generally allows more precise, faster and more regulated circulation, while an open system is structurally simpler and adequate for the metabolic demands of most invertebrates.

Define both circulatory types by whether blood stays confined to vessels, and match one phylum from the chapter to each.

• Reversing the two systems — remembering that Annelida (segmented worms) is closed while Arthropoda (jointed-limbed animals) is open is a common mix-up.
• Forgetting that cephalopod molluscs (e.g. Octopus) are a notable exception within the otherwise open-circulation phylum Mollusca.