Introduction & Significance of Coelom

Introduction :- 

A coelom is a fluid-filled body cavity found in most animals which is located in the mesoderm, derived from the splanchnic mesoderm, the middle germ layer only found in tiploblastic (three-layered) organisms. Simpler animals like Cniderians (ex- Jelly fish, Coral etc ) and sponges are diploblastic and monoblastic respectively, lacking a coelom. Though the coelom developed in tiploblastic animals, some of these animals have lost their coeloms.

Animals with a coelom, including the majority of animal phyla, are called coelomates. Animals without a coelom, such as flatworms, are called acoelomates. In between there are some animals called pseudocoelomates, which possess a "false coelom," which is an unlined or partially lined body cavity between the gut and body wall.

The earliest known animal with a coelom is vernanimalcula guizhouena, which lived 600 million years ago, during the Ediacaran period. The biggest disadvantage of lacking a coelom is that the internal organs are much more susceptible to compression and damage. The protection that coelomate animals get from their body cavity stems from the fact that fluids are incompressible, while organs are not. Since the organs are surrounded by an incompressible fluid, they are well-protected in coelomates.

The coelomates nonchordate phyla comprise Entoprocta, Ectoprocta, Phoronida, Brachiopoda, Mollusca, Priapulida, Sipuncula, Echiura, Annelida, Tardigrada, Pentastoma, Onychophora, Arthropoda, Pogonophora, Echinodermata, Chaetognatha, Hemichordata.

However, In contrast a lots of and numerous animals are pseudocoelomate, including nematodes, rotifers, kinorhynchans (mud dragon), nematomorphs (horsehair worms), gastrotrichs, loriciferans, priapulid worms, spiny-headed worms, and entoprocts. Many of these organisms are less than a millimeterin size, but some, like priapulid worms, grow as large as 6 inch (15 cm).

Significance :-

(i) A coelom is defined as a cavity that separates the gut from the body wall. The coelom allows the internal organs to shift around and develop independently of the body wall, creating more physiological and evolutionary flexibility. Though the coelom usually refers to the largest body cavity, coelomates animals may feature several strategically placed body cavities.

(ii) Organs formed inside a coelom can freely move, grow, and develop independently of the body wall while fluid cushions and protects them from shocks. Arthropods, mollusks even certain annelids have a reduced coelom. This condition in various ways may be compensated by other alternatives. As in former two groups principal body cavity is the hemocoel of an open circulatory system. In leeches the botrydoial tissue do the work as coelom is supposed to do.

(iii) In the past, zoologists grouped animals based on characters related to the coelom. The presence or absence of a coelom and the way in which it was formed was believed to be important in understanding the phylogenetic relationships of animal phyla. However, recent molecular phylogenies have suggested this characteristic is not as informative as previously believed : the coelom may have arisen twice, once in protostomes and once among the deuterostomes.

Formation :- 

Coelom formation begins in the gastrula stage. The developing degestive tube of an embryo forms as a blind pouch called the archenteron.

In Protostomes the coelom forms by a process known as schizocoely. The archenteron initially forms, and the mesoderm splits into two layers : the first attaches to the body wall or ectoderm, forming the parietal layer and the second surrounds the endoderm forming the visceral layer or alimentary canal. The space between the parietal layer and the visceral layer is known as the coelom or body cavity. In Deuterostomes, the coelom forms by enterocoely mesoderm buds from the walls of the archenteron and hollows to become the coelomic cavities.

Origins :- 

• The Acoelomate Theories :

 Coelom evolved from an acoelomate ancestor.

• The Enterocoel Theory :

Coelom evolved from gastric pouches of Cniderian ancestors.

Supported by research on flatworms and small worms recently discovered in marine fauna ("Coelom").