An animal that lacks a body cavity between the gut and the body wall, with organs embedded directly in solid tissue. Examples include flatworms and some other simple invertebrates.
From Greek 'a' (without) and 'koiloma' (cavity). The term emerged in the late 19th century as zoologists developed classification systems based on body cavity organization alongside other anatomical features.
Acoelomate animals are like living pancakes! Without a body cavity, they must stay flat so that nutrients and oxygen can reach all their cells by simple diffusion, which is why you'll never see a thick, chunky flatworm.
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