Body cavity
By the broadest definition, a body cavity is any fluid-filled space in a multicellular organism
Multicellular organism
Multicellular organisms are organisms that consist of more than one cell, in contrast to single-celled organisms. Most life that can be seen with the the naked eye is multicellular, as are all animals and land plants.-Evolutionary history:Multicellularity has evolved independently dozens of times...

. However, the term usually refers to the space located between an animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...

’s outer covering (epidermis) and the outer lining of the gut cavity, where internal organs develop. "The human body cavity
Human body cavities
The human body consists of the following body cavities:*dorsal body cavity**cranial cavity, enclosed by the skull and contains the brain, eyes, and ears.**spinal canal, enclosed by the spine and contains the spinal cord.*ventral body cavity...

" normally refers to the ventral body cavity
Ventral body cavity
The ventral body cavity is a human body cavity that is in the anterior aspect of the human body. It is made up of the thoracic cavity, and the abdominopelvic cavity. The abdominopelvic cavity is further separated into the abdominal cavity and pelvic cavity, but there is no physical barrier...

, because it is by far the largest one in volume.
The type of body cavity places an organism into one of three groups:
  • Coelomates (also known as eucoelomates — "true coelom") have a fluid filled body cavity called a coelom
    The coelom is a fluid-filled cavity formed within the mesoderm. Coeloms developed in triploblasts but were subsequently lost in several lineages. Loss of coelom is correlated with reduction in body size...

     (icon) with a complete lining called peritoneum
    The peritoneum is the serous membrane that forms the lining of the abdominal cavity or the coelom — it covers most of the intra-abdominal organs — in amniotes and some invertebrates...

     derived from mesoderm (one of the three primary tissue layers
    Germ layer
    A germ layer, occasionally referred to as a germinal epithelium, is a group of cells, formed during animal embryogenesis. Germ layers are particularly pronounced in the vertebrates; however, all animals more complex than sponges produce two or three primary tissue layers...