acoelomate

An acoelomate is an invertebrate that lacks a coelom, which is a fluid-filled body cavity. This body plan is one of the fundamental modes of organization for bilaterally symmetrical animals, meaning they have a distinct head and tail end and a left and right side.12

Acoelomates, such as flatworms and ribbon worms, have three tissue layers but no internal body cavity between the outer and inner layers. This lack of a coelom can limit the extent to which their muscles can contract and affects how efficiently they can supply tissues with oxygen and nutrients, often leading to relatively flat and small body sizes. Some acoelomates, like ribbon worms, may have a cavity at the head end for specific functions, but this is not considered a true coelom.21