EVOLUTION: THE BODY CAVITY

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 illustration of Nereis, above, shows the dorsal side opened above six of the parapodia (leg) bearing segments to show the coelom.  Nereis is an advanced polychete annelid, presumably more complicated than the ancestral one giving rise to the Pogonophora, but closer to the one giving rise to arthropods.

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

Isolation of organs for

physiological independence

          spatial independence when moving

                   minimal lubrication needed

          prevention of adhesions and

                   mouth formation and/or anastomoses

                   (a fundamental property of the underlying tissues

                 Of gut and blood vessels?)

Physiological independence is provided by the epithelium (peritoneum) covering the organs.  The covering provides a barrier to losing large molecules designed for function of the organ as well as preventing entry from those of other organs.  Thus the proper mix of chemicals needed for the specific role of the organ is not disrupted and/or is easier to maintain.  The barrier effect works because all coelomates have a circulatory system.  When a circulatory system is not present as in most pseudocoelomates the body cavity can provide many of its functions in supplying needed materials and removing wastes from the area for disposal by excretory and respiratory organs.

Spatial independence means the organs can reposition themselves without great distortion during movements.  For example, they don't get bent or squeezed so a gland is less likly to lose contol of providing its secretions appropriately.  The gentle activity they get from movements of the body is enough to help maintain normal function.  Activity is good for one's health if it is not too extreme.

The peritoneum covering the organs and lining the cavity are flattened cells making a smooth cover so that only a little moisture is needed for lubrication of movements.  Because the peritoneum is continuous from body wall to organ covering it forms a double layer or mesentary which provides two functions; it keeps the organs from getting twisted around or out of position and it can contain the blood vessels servicing the organ.  If an organ, such as kidney, is behind the peritoneum of the body wall, it  is said to be retroperitoneal in position; it then keeps its position better than if it were more flexibly located in a position just limited by its mesentaries. 

Prevention of adhesions is an important function because many tissues have the ability to fuse with other tissue if the peritoneum is not intact.  That is a property important in embryological development.  

The tissue of the primitive gut (the endoderm) of an embryo can fuse to the other layer (the ectoderm) and be the location of a mouth or an anus.  This property was important for the new location of the mouth to develop in pogonophoran ancestors as they moved into food-rich shallow seas and reform the digestive system as now seen in deuterostomes.

When the fusion and opening occurs as noted for the embryo it is fine.  But when surgical or other disruption of the peritoneum of adjacent organs occurs it can result in a new connection that is usually not wanted.  Such a structure connecting two parts of the digestive system is called a fistula. It may not be wanted, but is usually better than having an opening from the gut allowing contamination of the coelomic cavity, a typically fatal situation without rapid treatment.

Evolutionary considerations

The coelom was an effective way of solving some of the problems of increased body size.  Small organisms did not need special organs for overcoming the distance involved in moving all sorts of things used and produced by specialized organs.  Diffusion through fluids and tissues could take care of the problem.  But as size increased, direct diffusion was inadequate for moving things more than a few cells distance.  So to get adequate oxygen to tissues, some became flatter as they grew.  

Others (acoelomates) filled underlying spaces with jelly-like material that was nearly inert metabolically.  Others (pseudocoelomates) could get larger and have organs in the pseudocoel with enough fluid filling it to give them some escape from distortions with movement.  Also the fluid could move around and serve some of the transport role of a circulatory system.

Still other acoelomates had branches of the digestive system reaching to all regions of the body so movements of fluid in the gut could perform some of the vascular functions.  Hence, that type of development of the digestive system was called a gastro-vascular type of body cavity.

So coelom development was important when accompanying blood vascular system, respiratory, excretory, and endoskeletons needed for greatly increased size.  Of course there were other developments of importance.  One was the skin was no longer a single layer of cells or other simple covering; in the skin of vertebrates the epidermal layer is now serviced and supported by a dermal layer supplied with blood vessels and nerves.

When we talk about systems of the body, we seldom think of, or include, the coelom.  Too bad.

Four paragraphs back, I said "Others filled . . ."  -  is just a short way of saying "by the process of natural selection, survivors gradually, with the variants of the condition we are discussing, replaced the ancestral traits."

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.

Joseph G. Engemann     Emeritus Professor of Biological Science, Western Michigan University
Kalamazoo, Michigan    February 27, 2015