BODY CAVITY, THE COELOM
The coelom has some very important functions that are seldom discussed in biology books and classes.
The coelom, the body cavity containing some organs, would seem like it is almost nothing. But it is a nothing that is quite important in the evolution of animals. When the space around the internal organs is filled with large cells having little to do other than filling up the space, the organism is said to be acoelomate, or lacking a body cavity. When the same area is filled with fluid, the organism is said to be pseudocoelomate, or having a false body cavity. Those animals that are coelomate, a designation meaning they have a true body cavity, may not have much fluid in the cavity.
The evolutionary sequence was from acoelomate to pseudocoelomate to coelomate. The difference between a pseudocoelomate and a coelomate animal is the absence or presence of a epithelial lining or peritoneum which is present in a coelomate animal and absent in a peudocoelomate animal. The embryological origin of a true coelom varies. If it develops from a split in cells of the mesoderm (the middle layer of cells in an embryo, it is said to be schizocoelous. If it develops from outpocketing from the embryonic gut it is said to be enterocoelous.
IMPORTANT FUNCTIONS OF A COELOM
The views about the biology of the coelom expressed above are not based on experimental science in a direct way, but are more a view based on understanding of biological functions. A quick survey of a small variety of texts (evolution, animal physiology, vertebrate biology, and an invertebrate reference on smaller coelomate phyla) provided no functional discussion. The list under "important functions of a coelom" was in my journal notes for 10/03/06 that I read a few days ago and remembered talking to my classes about, in part, many years ago. I don't remember the lectures or articles or books that may have inspired it.
Kalamazoo, Michigan February 27, 2015