In certain animals like insects, an open body cavity where blood flows freely around organs instead of being enclosed in vessels.
From Greek 'haima' (blood) + 'koilos' (hollow). Biologists coined this term to describe this fundamentally different circulatory system found in invertebrates.
Insects have a haemocoel, which is why their blood (called hemolymph) just sloshes around their body cavity—no arteries or veins needed!
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