Coelom

What is a Coelom?

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The coelom is a body cavity filled with fluid that is found in animals and is located between the intestinal canal and the body wall. It forms from the three germinal layers during the embryonic development. The inner layer of the coelom is lined by the mesodermal epithelium cells. Animals are classified based on presence or absence of coelom and there’s another class of animals that have a pseudo coelom. Subsequently, the animals that possess true coelom are called coelomates and those who do not have the body cavity or coelom are called acoelomates.


Classification Based on Coelom

There are three such categories

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Acoelomate

These animals have no body cavity at all. Mesodermal tissues that are semi-solid hold the organs loosely in the blastocoel. Example- Flatworms, poriferans like sponges, etc.


Pseudocoelomate

These animals do have a true cavity or coelom. Hence, they are called pseudocoelomates. They have a false body cavity filled with fluid. Moreover, mesodermal cells partly line the blastocoel in these animals. The mesodermal layer is found only toward the body and not near the gut. These organisms also have organs held in place loosely and they are not in an organized fashion. Example- Roundworms, belonging to protostomes. Also, not all protostomes may be pseudocoelomates but all pseudocoelomates are protostomes.


Coelomate

They are also known as Eucoelomates or the organisms with the true body cavity- coelom. Their coelom has a complete mesodermal lining on both sides. Thus, the mesodermal layer is present on the body wall as well as in the gut. This lining is called peritoneum and it allows the organs to remain attached to each other so that they can be suspended in a particular order (in an organized manner) still being able to move freely within the cavity. During the stages of developments, the blastocoel in the gastrula gets replaced by the true coelom.

Example - Annelida, Chordata, Arthropoda, etc.


Types of Coelomates

Coelomates or Eucoelomates are of two types depending upon the embryonic development the organism goes through-

  • Protostomes - The coelom present is known as Schizocoelom.

  • Deuterostomes - The coelom present is known as Enterocoelom.


Types of Coelom

Depending upon the type of embryonic development, the coeloms formed in these organisms are also different. Protostomes develop Schizocoelom and Deuterostomes develop Enterocoelom.


Schizocoelom 

In these organisms the coelom originates from the splitting of mesoderm. It is found in annelids, arthropods, and molluscs. One part of the mesoderm attaches to the ectoderm and the other surrounds the endoderm. And the space between the two develops into a coelom. Coeloms of arthropods and Mollusca are filled with blood and they are called hemocoels.


Enterocoelom

The coelom forms by the fusion of internal outgrowths of the archenteron. It pinches off and fuses and is lined by a mesodermal layer. Echinodermata and Chordates fall in this category.


Functions of Coelom

  • Coelom acts as a protective layer that absorbs shocks. It absorbs most of the mechanical shocks. This cavity also provides flexibility to the organs to move. It also gives cushioning effects to the internal organs from damage on minor bends.

  • The coelomic fluid is a hydrostatic skeleton that helps in locomotion for soft-bodied animals. It also gives the body a definite shape. Contracting muscles can push against the coelomic fluid as a result of the fluid pressure.

  • The cells of the coelom called coelomocyte cells either float freely in the coelom or remain attached to the wall. The function of these cells is to support the immune system. They initiate the humoral immune response and phagocytosis.

  • Another function of the coelomic fluid is transporting gases, nutrients, and waste products.

  • Coelom provides extra space to the organs to develop properly and carry out their functions. Coelom makes the pumping of the heart possible, and also makes room for the baby in the womb.

FAQ (Frequently Asked Questions)

1. What is Coelom?

The coelom is a body cavity filled with fluid that is found in animals and is located between the intestinal canal and the body wall. It forms from the three germinal layers during the embryonic development. The inner layer of the coelom is lined by the mesodermal epithelium cells. Animals are classified based on presence or absence of coelom and there’s another class of animals that have a pseudo coelom. Subsequently, the animals that possess true coelom are called coelomates and those who do not have the body cavity or coelom are called acoelomates.

2. Are Human Beings Coelomates?

Yes, human beings possess a coelom and are coelomates. The coelom in the human body segregates into many unconnected cavities during the developmental stages. Although the coelomic cavity is not found in adult human individuals it is seen in embryonic stages. The coelom in various embryonic phases splits into several other cavities.

3. Do Sponges Have a Coelom?

No, sponges are not coelomates. They are poriferans with a large body cavity exposed to the outside that enables them to consume food. Whereas, the cavity of coelom holds vital organs like heart, lungs, kidneys together and seals it off from outside contact.

4. Are Annelids Coelomates?

Yes, Annelids are coelomates and possess a true coelom. They are bilaterally symmetric with a high degree of cephalization. Their internal body cavity is bound by mesodermal membranes. And the coelom is a fluid-filled cavity holding the internal organs in a loose framework.