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When we talk about the origin of life as we know it, the coelom is an important part that depicts the evolution and adaptation of animals since the beginning of time. Coelom is derived from a Greek word meaning hollow. It is an important part of the animal structure that helps us understand the functioning of the earliest animals to the most advanced ones. 

Table of Content 

  • What is coelom?
  • Types of coelom 
  • Types of organisms based on coelom 
  • Functions of coelom 
  • Practice problems 
  • FAQs 

What is Coelom?

The coelom is a fluid-filled body cavity that exists in animals, situated between the intestinal canal and the body wall. It develops through the three germinal layers in embryonic development. 

The coelom is lined with a thin layer of tissue known as the mesothelium, which produces a lubricating fluid that enables organs to move around and glide past each other without resistance. The internal organs are cushioned by this chamber of fluid, which serves to insulate them from outside influences and provide support.

The coelom has been essential for the evolution and diversity of the animal kingdom. Its existence in different animal species has facilitated the differentiation of organ systems and the evolution of increasingly sophisticated bodies. From earthworms to mammals, the coelom has given an outline for the evolution of several animal phyla. 

The structure of coelom

Image: The structure of coelom

Types of Coelom 

There are two types of coelom found in organisms: 


A schizocoelom is a body cavity divided along its middle by a septum. This form of the body cavity is prevalent in invertebrates, including worms and arthropods. The cavity is split in half by the septum, letting the animal adjust the size of its body cavity as necessary.

Whenever embryonic cells divide irregularly, a hollow cavity called a schizocoelom develops in the body of the developing organism. The ectoderm, the outermost layer of cells, is formed from one group of cells, and the endoderm, the innermost layer, is formed from a different group of cells. The schizocoelom is created in the gap between these two levels.


An enterocoelom is a body cavity formed when the mesoderm separates into two layers. The cavity is situated in the embryo's core and is enveloped by endoderm and ectoderm. The enterocoelom creates an area for the embryo to multiply and develop. Flatworms and ribbon worms both have this body arrangement.

Types of Organisms Based on Coelom 

On the basis of the presence of a coelom, there are three types of organisms: 


An acoelomate comprises a body cavity that does not consist of organs. There is no coelom present in acoelomates. The blastocoel is entirely covered by mesoderm. The characteristics of acoelomate animals include having a simple and smooth body exterior. 

The body cavity of an acoelomate is unlined by cells. Since they lack a digestive system, they process food outside of their own bodies. Animals that are acoelomates include ribbon worms and flatworms. 


Image: Acoelomate 


As the name suggests, there is no true coelom present in pseudocoelomates. Mesodermal cells constitute some of the blastocoel's structure. 

Mesoderm lines the body cavity entirely in the direction of the body wall; it does not extend into the digestive tract. Additionally, the organs of these organisms are not organised and are only attached loosely in position. Earthworms and nematodes are two examples of pseudocoelomate creatures.


Image: Pseudocoelomate

Eucoelomate or Coelomate

A coelomate or eucoelomate's body cavity is segmented into sections by membranes called septa. The animal is capable of shifting the internal organs throughout the fluid-filled sections without injuring them. Additionally, this internal bodily fluid enables oxygen and nutrients to circulate to every region of the body.

An acoelomate is a living creature whose body cavity is entirely lined with tissue developed from the mesoderm. The coelom is formed when the developing embryo separates into three distinct layers:

  • The ectoderm, which is the outer layer, develops into the skin and nervous system
  • The mesoderm, the middle layer, forms the basis of the circulatory system, bones, and muscles
  • The endoderm, the inner layer, develops into the digestive system and respiratory systems

The organisms of Phylum Annelida to Chordata are coelomates. 


Image: Eucoelomate 

Functions of Coelom 

Some of the functions of coelom are as follows: 

1. In certain organisms, the coelom fills with fluid, thereby helping in supporting and cushioning the organs.

2. Coelom functions as a cushioning layer which absorbs shocks. It absorbs the majority of the mechanical shocks. This area also allows the internal organs to move around more freely. Additionally, it protects internal organs against injuries during minimal bending by acting as a cushion.

3. For soft-bodied animals, the coelomic fluid functions as a hydrostatic framework for helping with locomotion. Additionally, it provides the body with a distinct contour. The fluid pressure caused by muscles relaxing can cause them to push backwards on the coelomic fluid.

4. Coelomocyte cells may swim easily in the coelom or stay anchored to the wall. These kinds of cells function to support the body's immune system. In addition to phagocytosis, they begin the humoral immune system's response.

5. The coelomic fluid acts as a medium for the movement of food, oxygen, and metabolic byproducts within the body. It makes it easier for chemicals to move back and forth across tissues and organs, ensuring that vital nutrients are distributed, and metabolic waste is eliminated.

6. The coelom is essential for preserving body form and framework in creatures without a solid skeletal system. Internal support is provided by the coelomic fluid's hydrostatic pressure, which enables the organism to keep its shape and move.

Practice Problems 

Q1. The true coelom is present in between

A. Body wall and ectoderm 
B. Endoderm and mesoderm 
C. Body wall and ectoderm 
D. Mesoderm and ectoderm 

Ans. B. The coelom is the body cavity which is present between the body wall and the mesoderm layer of the organism. 

Q2. When the mesoderm splits, the body cavity formed is called?

A. Pseudocoelom 
B. Blastocoel
C. Haemocoel
D. Coelom 

Ans. D. Coelom is the body cavity filled with fluid which is formed by the splitting of mesoderm. 

Q3. Which of the following organisms is a pseudocoelomate?

A. Annelids 
B. Cnidaria 
C. Nematodes
D. Flatworms

Ans. C. The organisms of the phylum Nematoda have a pseudocoelom, and the organs are loosely packed in these organisms. 


Q1. Do insects have a coelom?
Ans. Yes, insects such as earthworms, snails, etc., are eucoelomates, meaning they have a coelom. 

Q2. Do hydras have a coelom?
Ans. Hydras do not have a coelom and are acoelomates. 

Q3. What is coelom derived from?
Ans. The coelom is derived from the mesodermal germ layer of the vertebrates.

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