Acoelomate

An acoelomate is an animal whose body lacks a fluid-filled coelom between the digestive tract and body wall. Space between these structures is occupied by tissue.

Coelom: A fluid-filled body cavity completely lined by mesoderm, located between the digestive tract and body wall. Its absence is the defining feature that makes an animal acoelomate.
Flatworm: An invertebrate with a flattened, usually unsegmented body belonging to the phylum Platyhelminthes. Flatworms are the familiar examples of animals traditionally classified as acoelomates.
Diffusion: The net movement of particles from regions of higher concentration to regions of lower concentration. Many acoelomates depend on short diffusion distances for gas and nutrient exchange.
Pseudocoelomate: An animal with a pseudocoel, a body cavity that is not completely lined by mesoderm. Pseudocoelomates have an internal cavity that acoelomates entirely lack.
Pseudocoelom: A body cavity between the digestive tract and body wall that is not completely lined by mesoderm. It distinguishes pseudocoelomate animals from acoelomates, which lack this cavity.
Planarian: A free-living flatworm known for its strong regenerative capacity and simple nervous system. Its flattened body has no coelom and relies on diffusion for internal exchange.
Gastrovascular cavity: A digestive compartment with a single opening that also distributes nutrients through the body. In many flatworms, branching gut spaces help distribute nutrients without a coelomic transport system.
Coelomate: An animal with a coelom, a body cavity completely lined by mesoderm. Coelomates contrast directly with acoelomates in the presence of a mesoderm-lined cavity.
Mesoderm: The embryonic germ layer that gives rise to muscles and many internal tissues and organs. Mesoderm fills the space between the gut and body wall in acoelomates.
Tapeworm: A parasitic flatworm whose adult body absorbs nutrients through its outer surface. Tapeworms retain the acoelomate flatworm body plan despite their specialized parasitic lifestyle.

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