Coelom

A fluid-filled body cavity completely lined by mesoderm. It provides space for internal organs and, in many animals, supports movement and organ development.

Mesoderm: The middle embryonic germ layer, which forms muscles, connective tissues, and many internal organs. Mesoderm completely lines the coelom and forms much of its surrounding tissue.
Schizocoely: A mode of coelom formation in which spaces split within mesodermal tissue. This is one developmental route by which a coelom forms.
Acoelomate: An animal whose body lacks a fluid-filled cavity between the digestive tract and body wall. Acoelomates have no coelom separating their organs from the body wall.
Annelida: The animal phylum comprising segmented worms, including earthworms and leeches. Many annelids have a spacious coelom partitioned by internal septa.
Germ layer: An embryonic cell layer that gives rise to particular tissues and organs. The coelom’s defining lining comes from one of the embryo’s germ layers.
Enterocoely: A mode of coelom formation in which pouches of the embryonic gut become body cavities. This alternative route forms coelomic spaces from the developing gut.
Pseudocoelom: A body cavity not completely lined by mesoderm, often derived from the embryonic blastocoel. Its incomplete mesodermal lining distinguishes it from a true coelom.
Mollusca: The animal phylum comprising snails, clams, squids, and their relatives. Mollusks retain a reduced coelom around organs such as the heart and kidneys.
Blastocoel: The fluid-filled cavity inside the early embryo’s blastula stage. Unlike a coelom, the blastocoel is not completely lined by mesoderm.
Peritoneum: A serous membrane lining the abdominal cavity and covering many abdominal organs in vertebrates. It is a mesoderm-derived membrane associated with the vertebrate coelomic cavity.

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