What is Coelom?

The coelom is one of the characteristic features of metazoans. The true coelom is a body cavity formed during embryo development from the three germinal layers. The coelom is lined by mesodermal epithelium cells. Presence or absence of coelom is one of the criteria for classifying animals.

The animal kingdom is divided into three groups on the basis of the nature of coelom.

Coelom Structure

Animal Kingdom

Acoelomate

(no coelom)

Pseudocoelomate

(with false coelom)

Coelomate

(with true coelom)

Porifera

Coelenterata

Ctenophora

Platyhelminthes

Aschelminthes

Annelida

Arthropoda

Mollusca

Echinodermata

Hemichordata

Chordata

Coelom Definition

The coelom is the fluid-filled body cavity present between the alimentary canal and the body wall. The true coelom has a mesodermal origin. It is lined by mesoderm. The peritoneal cavity present in the abdomen and similar spaces around other organs such as lungs, heart are parts of the coelom. Coelom differs in its structure and formation process.

Structure, Formation and Types of Coelom

There are three types of structural body formation present in animals related to coelom:

types of coelom

1. Acoelomate:

Coelom is absent. The blastocoel is completely occupied by mesoderm. E.g. Porifera, Coelenterata and Flatworms (Platyhelminthes). There is only spongocoel or coelenteron present.

2. Pseudocoelomate:

True coelom is not present. The blastocoel is partly filled by mesodermal cells. The body cavity is lined by mesoderm only towards the body wall and mesoderm is not present towards the gut. E.g. Roundworms (Aschelminthes)

3. Eucoelomate:

Animals that have a true coelom. The coelom is lined by mesoderm on both the sides, towards the body wall and towards the gut. The blastocoel present in the gastrula gets completely replaced by a true coelom. The body organs are suspended in the coelom by mesenteries. E.g. from the phylum Annelida to Chordata.

Eucoelomates are further divided into Protostomes and Deuterostomes on the basis of different embryonic development. The process of coelom formation in protostomes and deuterostomes is different. The coelom is categorised into two types on the basis of formation, namely, Schizocoelom and Enterocoelom.

Schizocoelom- Protostomes Eneterocoelom- Deuterostomes

  1. Schizocoelom: It is present in the protostomes. The body cavity or coelom originates from the splitting of mesoderm. One part attaches to ectoderm and the other surrounds the endoderm. The space between them develops into the coelom. The blastopore forms mouth.

    Examples of schizocoelous animals are Annelida, Arthropoda and Mollusca. In Arthropoda and Mollusca the coelom is filled by blood and is known as Haemocoel.

  2. Enterocoelom: It is present in the deuterostomes. The coelom is formed from the fusion of the internal outgrowths of the archenteron, that pinches off and fuses together to form coelom lined by mesoderm.

    Examples of enterocoelous animals are Echinodermates and Chordates.

Coelom Function

  • Coelom works as a shock absorber and protects from any kind of mechanical shock. It gives more flexibility to the body organs to move and protects from any damage on minor bends by cushioning the internal organs
  • The coelomic fluid acts as a hydrostatic skeleton, which helps in the locomotion of soft-bodied animals and gives the body a definite shape. Contracting muscles can push against the coelomic fluid because of the fluid pressure.
  • The coelomocyte cells, that either float freely in the coelom or attached to the wall, support the immune system. They support the immune system by initiating humoral immune response and phagocytosis
  • The coelomic fluid also helps in gaseous transport and transport of nutrients and waste products
  • Coelom gives the extra space required by organs to develop and function. E.g. pumping action of the heart, carrying a child in the womb, etc. is possible due to coelom

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