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Coelom is a body cavity that is lined by mesoderm epithelial cells. It is formed during embryo development from the three germinal layers.
- Coelom is a fluid-filled body cavity present between the intestinal tract and the body wall.
- Coelom consists of the peritoneal cavity present in the abdomen and similar spaces around organs such as the heart and lungs.
- And it is one of the characteristic features of metazoans. A true coelom is lined by mesoderm and has a mesodermal origin.
- Animals are classified based on the presence or absence of coelom. Certain animals have a false coelom.
- The animals with true coeloms are called coelomates, while the ones that do not are called acoelomates.
Some animals that have a coelom are annelids, arthropods, molluscs, etc. Such animals are called coelomates. One important function of Coelom is to absorb shock and protect the internal organs of the body.
Read Also: Biodiversity
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Key Terms: Coelom, Acoelomate, Pseudocoelomate, Coelomate, Protostomes, Deuterostomes, Schizocoelom, Enterocoelom
What is Coelom?
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Coelom is a fluid-filled body cavity located between the intestinal tract and the body wall. It is the main body cavity in some animals. A true coelom originates from mesodermal cells and is lined by them.
- During embryonic development, the coelom is formed from the three germinal layers.
- Based on the presence or absence of coelom, animals can be classified.
- Animals with a true coelom are called coelomates, while those that do not possess a coelom are called acoelomates.
- There are some other animals that have a false coelom and are called pseudocoelomates.
Coeloms are of three types. They are:
| Type | Acoelomate (No Coelom) | Pseudocoelomate (False coelom) | Coelomate (True Coelom) |
|---|---|---|---|
| Examples | Ctenophora, Porifera, Platyhelminthes, Coelenterata | Aschelminthes | Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata, Chordata |
Coelomic Fluid
The fluid found inside the coelom is called coelomic fluid. It is typically circulated by mesothelial cilia or by muscle contraction in the body wall that are composed of mesin. The coelomic fluid performs multiple functions.
For example,
- It acts as a hydroskeleton, thus allowing free movement and growth of internal organs.
- It also acts as a medium to transport gases, nutrients and waste products around the body.
- It allows sperm and egg storage during maturation.
- The Coelomic Fluid also acts as a reservoir for waste.
Classification of Coelom
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Based on the nature, structure, and body formation of the coelom, animals are classified into three categories.
Acoelomate
In acoelomates, the coelom is completely absent. In such animals, the blastocoel is completely occupied by the mesoderm. They only have spongocoel or coelenteron. Some examples of coelomates are
- Porifera
- Coelenterata
- Ctenophora
- Platyhelminthes (also known as, flatworms)
Pseudocoelomate
Pseudocoelomates are animals that do not have a true coelom. The coelom present is a false coelom. In such animals, the blastocoel is partly filled by mesodermal cells. The mesoderm lines the body cavity only towards the body wall and is not present towards the gut. An example of a pseudocoelomate is Aschelminthes (roundworms).
Coelomate or Eucoelomate
Coelomates are animals that have a true coelom. A true coelom has a mesodermal origin and is lined by mesoderm on both sides, towards the body wall and the gut. In such animals, the blastocoel, which is present in the gastrula, is completely replaced by the true coelom. Here, the body organs are suspended in the coelom by mesenteries.
Some examples of coelomates are:
Types of Coelom
Types of CoelomateBased on embryonic development, the coelomates are classified further into protostomes and deuterostomes. They both have a different process of formation of a coelom.
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Also Read: Open-Type Circulatory System
Types of Coelom
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Based on the formation of the coelom, it is classified into two types: Schizocoelom and Enterocoelom.
Schizocoelom
Schizocoelom is present in protostomes. The origin of the body cavity, or coelom, occurs due to the splitting of the mesoderm. Of which, one part surrounds the endoderm and the other attaches to the ectoderm. The space between these two forms a coelom, and the blastopore forms the mouth.
Some examples of animals that consist of a Schizocoelom are:
- Annelida
- Mollusca
- Arthropoda
In Mollusca and Arthropoda, the coelom is known as the hemocoel, as it is filled with blood.
Enterocoelom
Enterocoelom is present in deuterostomes. Here, the coelom is formed by the fusion of the internal outgrowths of the archenteron. It fuses and forms a coelom lined by mesoderm.
Some examples of animals that have Enterocoelom are:
- Chordates
- Echinoderms
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Functions of Coelom
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Some functions of coelom are listed below:
- Coelom protects the body against any kind of mechanical shock and works as a shock absorber. It acts as a cushion for internal organs and protects them against damage from minor bends and provides flexibility to them.
- The coelomic fluid helps in the locomotion of soft-bodied animals and maintains a definite body shape. It acts as a hydrostatic skeleton for the body.
- The coelomocyte cells are either attached to the wall or float freely in the coelom. They provide support to the immune system by initiating humoral immune responses and phagocytosis.
- Another function of the coelomic fluid is to help with gaseous transport and transport of nutrients and waste products.
- Coelom provides enough space for organs to develop and function.
Significance of Coelom
The coelom plays a key role in the life of animals. Some of which are:
- The coelomic fluid content helps in the smooth transportation of particles or materials in a solution.
- Coelom aids flexibility to the body, thus extending room for gut movement that is suspended.
- Gonads that grow from the coelomic epithelium can be found in the cavity of the coelom. So are the nephridial tubules, that connect the coelom to the exterior, allowing passage of eggs and sperm.
- The coelom is filled with incompressible coelomic fluid, serving as a hydrostatic skeleton and aiding locomotion.
Development of Coelom
Also Check: Embryology
Things to Remember
- Coelom is a body cavity that is surrounded by mesodermic cells.
- The presence or absence of a coelom forms a basis for the classification of animals.
- Animals are classified into three categories based on the coelom: acoelomate, pseudocoelomate, and coelomate.
- Based on how a coelom is formed, it can be of either of the two types: schizocoelom or enterocoelom.
- Coelom protects the internal organs of the body and also works as a shock absorber.
Also Check:
Previous Year Questions
- A well-developed archegonium with a neck consisting of 4-6 rows... (NEET 1995)
- The antherozoids of Funaria are... (NEET 1999)
- In the classification of plants, the term cladistics refers to ... (KEAM)
- The heterosporous pteridophyte belonging to the class lycopsida is... (KEAM)
- In Pinus; the pollen grain has 6 chromosomes then in its endosperm... (NEET 1992)
- Dichotomous branching is found in (NEET 1999)
- Kingdom Animalia is characterised by... (AIIMS 2011)
- Select the correct statement... (NEET 2016)
- Scyphozoan medusae are commonly known as... (COMEDK UGET 2010)
- External fertilization occurs in majority of... (DUET 2009)
Sample Questions
Ques: Are human beings coelomates? [1 mark]
Ans: Yes, human beings are coelomates and possess true coelom. While developing, the coelom segregates into various unconnected body cavities. Adults do not possess this cavity. But during the embryonic phase, it splits into other cavities.
Ques: Do sponges have a coelom? [1 mark]
Ans: No, sponges do not have a coelom. A coelom is a body cavity that seals internal organs like the heart, lungs, kidneys, etc. from outside contact, and sponges are poriferans that lack a coelom. Their body cavities are huge and exposed towards the outside to enable food consumption.
Ques: Are annelids coelomates? [1 mark]
Ans: Annelids have a true coelom, so they are coelomates. Their internal body cavities are covered in mesodermal cells. They are bilaterally symmetric, triploblastic, and have a degree of cephalization. Internal organs form inside the coelom of an annelid. Annelids are also among the first organisms to have a coelom.
Ques: Do molluscs have a coelom? [1 mark]
Ans: Molluscs are triploblastic and bilaterally symmetric and have a true coelom. The coelom is a body cavity that is surrounded by mesodermic cells. But the coelom of molluscs is reduced to a hemocoel, which is a tiny void near the heart. Molluscs are among the first organisms to have a coelom.
Ques: Explain the different types of coelom. [2 marks]
Ans: Coelom is of two types based on its formation, schizocoelom and enterocoelom.
- Schizocoelom: Schizocoelom originates due to the splitting of the mesoderm. One of its parts attaches to the endoderm and the other to the ectoderm. The space between them forms a coelom, and the blastopore forms the mouth. It is present in protostomes. Some animals that have schizocoelom are Mollusca, Arthropoda, and Annelida. In Arthropoda and Mollusca, the coelom is filled with blood and is known as the haemocoel.
- Enterocoelom: Enterocoelom is formed by the fusion of the internal outgrowths of the archenteron. They fuse and form a coelom lined by mesoderm. Enterocoelom is present in deuterostomes. Some animals with enterocoely are chordates and echinoderms.
Ques: Write a brief note on coelomates. [2 marks]
Ans: Animals with a true coelom are called coelomates. The coelom has a mesodermal origin. And it is lined by mesoderm on both sides, the gut, and the body wall. The blastocoel present in the gastrula of such animals is replaced completely by the true coelom. And the body organs are suspended in the coelom by mesenteries. Some examples of coelomates are Mollusca, Arthropoda, Annelida, and Chordata. Coelomates are classified into two types, protostomes, and deuterostomes, based on the process of their formation.
Ques: What is Coelom? [3 marks]
Ans: Coelom is the main body cavity in most organisms.
- It is a hollow, fluid-filled cavity that serves as a hydrostatic skeleton and envelops the internal organs. It is located between the intestinal tract and the body wall in animals. And it is formed during embryonic development from the three germinal layers.
- The inner layer of a coelom consists of mesodermal cells. Animal classification is also done based on the presence or absence of a coelom in them. And certain animals have a pseudo-coelom.
- Animals with a true coelom are called coelomates, and animals with no coelom are called acoelomates.
Ques: State the importance of the study of body cavity and coelom in the classification of animals. [5 marks]
Ans: A coelom is a fluid-filled body cavity between the body wall and the intestinal tract.
- The presence or absence of a coelom is an important aspect of the classification of organisms.
- Animals that contain a true coelom are called coelomates. Some coelomates are molluscs, annelids, arthropods, etc.
- Animals that do not have their body cavities lined up by mesoderms are called pseudocoelomates.
- They have mesoderms scattered between the ectoderm and endoderm.
- An example of a pseudocoelomate is Aschelminthes.
- In certain animals, the coelom is completely absent, and they are known as acoelomates.
- Some examples of acoelomates are Coelenterata, Ctenophora, Porifera, etc.
Ques: Explain the classification of animals based on coelom. [5 marks]
Ans: Classification of animals can be done based on the presence or absence of a coelom in them. There’s a certain category of animals that don’t fit into either of the two, as they possess a false coelom. So, the animals are classified into acoelomates, coelomates and pseudocoelomates.
- Acoelomates = Acoelomates are animals that do not possess a coelom. In such animals, as the coelom is completely absent, the blastocoel is completely occupied by the mesoderm. They only possess spongocoel or coelenteron.
Examples of acoelomates: Coelenterata, Porifera, Flatworms
- Coelomates = Animals that possess a true coelom are called coelomates. A true coelom is lined by mesoderm on both sides, the gut and the body wall. The blastocoel in such animals is completely replaced by the true coelom. The body organs here are suspended in the coelom by mesenteries.
Examples of coelomates: Annelida, Arthropoda, Molluscs
- Pseudocoelomates = Animals that do not fall into either of the above two categories because of the presence of a false coelom are called pseudocoelomates. Here, the blastocoel is partially covered by mesodermal cells. Mesoderms are present only on one side of the body cavity, which is the body wall, and are not present near the gut.
An example of pseudocoelomate: roundworms
Ques: What are the functions of a coelom? [5 marks]
Ans: Coelom has numerous functions, and they are listed below:
- It acts as a cushion for the internal organs of animals and protects them from external damage.
- It also allows the internal organs to grow, change shape and move independently within the outer body wall.
- In organisms such as worms, it helps to maintain pressure inside the body. It acts as a hydrostatic skeleton for the body.
- It works as a shock absorber and protects the body against mechanical shocks.
- Coelomic fluid helps transport nutrients, waste products, and gases.
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