Aceolomates

Aceolomates, also spelled acoelomates, are animals that lack a true coelom, the fluid-filled body cavity formed within the mesoderm. In aceolomates the middle tissue mass is a solid parenchyma, and there is no body cavity between the gut and the body wall.

Most aceolomates are bilaterally symmetrical and generally dorsoventrally flattened. They are typically triploblastic, possessing ectoderm, endoderm, and mesoderm, but have comparatively simple organ systems relative to coelomates. The digestive system varies among groups: many free-living flatworms (Platyhelminthes) have a gastrovascular cavity with a single opening; others have highly reduced or absent alimentary canals and absorb nutrients across the body surface. Excretory and nervous systems are also relatively simple; many possess protonephridia for osmoregulation and a ladder-like nerve net or simple nerve cords.

Taxonomically, the term traditionally encompassed flatworms (Platyhelminthes) and the small, worm-like Acoelomorpha. Some modern classifications place Acoelomorpha in a basal position relative to other bilaterians, which can complicate higher-level grouping, but the core concept remains: aceolomates do not possess a true coelom.

Reproduction is often hermaphroditic; many planarians can reproduce asexually by fission, while cestodes and trematodes exhibit more complex lifecycles and reproductive modes. They inhabit a range of environments, including marine, freshwater, and moist terrestrial habitats; some are free-living predators or scavengers, while others are endoparasites. The study of aceolomates informs discussions on body plan evolution, development, and the diversity of parasitic life strategies.