Coelom

Cutting an oligoqueto worm, a protostomio Annelida.

The coelom is the secondary general body cavity of coelomates or pseudocoelomates. It is said to be general because it does not communicate with the external environment, which is not strictly true, above all in the case of enterocelia, and it is said to be secondary because, as such a general cavity, it is the second to appear, after the blastocoel, throughout embryonic development.

The coelom is of mesodermal origin, therefore it is exclusive to triblastic animals; it is limited by mesodermal epithelium called peritoneum and filled with coelomatic fluid, which performs different functions. The internal organs remain outside the coelom and are surrounded by the peritoneum.

In adults, the coelom persists in highly variable ways. The coelom can be preserved in the cavities of the body segments, then acting as a hydrostatic skeleton, in the lumen of the gonads and gonoducts (and then it receives the name of gonocele), in the lumen of the excretory organs, such as the nephridia (nephrocele), in the pericardial cavities, as well as in the lumen of certain circulatory systems.

Training

In the embryos of protostomous animals (annelids, molluscs, arthropods), the mesoderm and the coelom originate from schizocelia; Cells located at the junction of the endoderm and ectoderm proliferate and migrate into the blastocoel and form the mesoderm, from which the coelom forms. These animals are called schizocoelomates.

Classification of animals according to the presence/absence of coelom

Not all triblastic animals (i.e. with mesoderm) develop a coelom:

• Acelomados. A triblastic animal in which jealousy is not formed, receives the name of acelomado animal; possesses, of course, mesoderm, but this is solid. Silver-mint worms are aceloomed.

• Pseudocellomados. There are triblotic animals with a body cavity similar to the lattice in function, but very different embryologically since it is not of mesodermic origin; such cavity is called pseudoceloma or blastoceloma and the animals that possess it are called pseudocelomados or blastocellomados; this cavity is usually a persistent embryonic blastocele and, unlike the authentic perineum Typically pseudocellomads are nematode worms and rotmals.

• Jealous. Finally, the animals with true jealousy are called jealous or ecchylomids ("authentic cellomates"). They are basically all animals, except for certain very simple groups (poriferous, cnidaries, platelmintos, nematodes, rotmals...).

Functional significance of the coelom

The existence of the coelom enabled the evolution and diversification of large animal groups: annelids, arthropods, vertebrates and mollusks, thanks to the fact that body compartmentalization helped the different systems and organs of the body to have a better division of tasks, with its consequent specialization.

The acquisition of a coelomic cavity is considered of vital importance for the subsequent evolution of the body organization of metazoans. This is because the coelom makes it possible to fully exploit the functional possibilities of a fluid-filled body cavity, some of which already appeared in the pseudocoelomate organization. Among these functions, the following should be highlighted:

• The cell fluid acts as a transport system between the wall of the body and that of the digestive tract, facilitating the distribution of nutrients, metabolites and respiratory gases and being able to temporarily store waste products to be excreted later.

• This transport system has obvious advantages over the self-distributing transport of acelomados. As a result of the increased physiological effectiveness of the body, it can increase in size, with the evolutionary implications that this means: better homeostasis and independence of the environment, diversification of available sources of food, etc.

• The cavity filled with fluid confers independence on the organs, favoring physiological processes as basic as the independent heartbeat or production, in vermiform organisms, of inverse peristaltic waves on the wall of the body and the digestive tract, which counts on the jealousy with its own musculature.

• As there is a common space that collects and circulates the products of the metabolism, the repeated structures, which in the acelomados were supplied by diffusion, can now concentrate on one or two more effective and complex organs.

• The jealous fluid can act as an internal hydrostatic skeleton, reminiscent of a water-filled balloon.