The liquid filled cavity between alimentary canal and body wall of multicellular triploblastic animals is known as body cavity. Generally, the body cavity that is lined by mesodermal peritoneal membrane is known as coelom. According to Hyman (1955), coelom is the hollow space between alimentary canal and body wall which is lined by mesodermal peritoneal tissues. Coelom gives the space for most of the visceral organs. Between two embryonic layers of mesoderm, coelom originates as a secondary body cavity. Haeckel first proposed the term coelom in 1972.
Coelom is also known as the perivisceral cavity that has fluid filled compartments, transversely partitioned by septa. In arthropods and mollusks, coelom is reduced or absent but it is present in the embryonic stage. Instead of coelom, they have space with blood and lymph, known as haemocoel. In these animals, the haemocoel is the primary body cavity while the coelom is the secondary body cavity.
Special Features of Coelom
- Coelom is a secondary body cavity which is formed during embryonic development by splitting of mesoderm into two layers, a somatic layer and a splanchnic layer.t
- Coelom is surrounded by the coelomic epithelium.
- Reproductive organs arise from the walls of the coeloimic epithelium.
- The perivisceral cavity or splanchnocoel is formed from the greater part of the coelom.
- Coelom is filled with colorless coelomic fluid which provides space to house the viscera.
- The excretory organs also open into the coelom.
Types of Coelom
There are the following three types of coelom, such as:
- Acoelom
- Pseudocoelom
- True Coelom or Eucoelom
Image showing different Coelom
Acoelom
In this case, coelomic fluid-filled cavity is absent but the space between the body wall and the gut is filled by connective tissue, known as mesenchyme. The animals without a coelomic fluid-filled body cavity between the body wall and digestive tract are known as acoelomates and the group of animals is referred to as acoelomata.
Examples: Acoelomata Phyla include:
- Gnathostomulida,
- Platyhelminthes and
- Nemertea,
- Gastrotricha,
- Cnidaria
- ctenophora
- Kinorhyncha.
Pseudocoelom
In this case, coelom is not originated by the mesodermal epithelium. Embryologically, the pseudocoelom is developed from the blastocoel of the embryo. The pseudocoelom is also known as false coelom. The pseudocoelom is not bounded by the peritoneum. The fluid of pseudocoelom contains pseudocoelocytes which has no relation with the reproductive and excretory organs. Animals with a pseudocoelom are known as pseudocoelomates and the group of animals is referred to as pseudocoelomata. Pseudocoelomata are also referred to as blastocoelomata or haemocoelomata.
Examples: According to Brusca and Brusca (2003), pseudocoelomata phyla include:
- Rotifera
- Kinorhyncha
- Nematoda
- Nematomorpha
- Acanthocephala
- Loricifera
Coelom
In this case, the body cavity between the gut and body wall is bounded by peritoneum. Peritoneum is derived from the embryonic mesoderm. This type of cavity or coelom is known as eucoelom or true coelom. The coelomic fluid of true coelom contains amoebocytes. The animals with true coelom are known as coelomates and the group of animals is referred to as coelomata.
Examples: According to Brusca and Brusca(2003) the following phyla possess a true coelom:
- Annelida
- Onychophora
- Tardigrada
- Arthropoda
- Mollusca
- Phoronida
- Ectoprocta
- Brachiopoda
- Echinodermata
- Chaetognatha
- Hemichordata
- Chordata
Some Similarities Between Coelom and Pseudocoelom
- Both coelom and pseudocoelom contain coelomic fluid.
- Peritoneum is present in both coelom and pseudocoelom.
- Both coelom and pseudocoelom are present in triploblastic animals that possess three germinal layers such as ectoderm, mesoderm and endoderm.
- Both coelom and pseudocoelom contain alimentary canal.
- Both coelom and pseudocoelom act as a hydroskeleton.
Difference between Coelom and Pseudocoelom
The following table shows the differences between coelom and pseudocoelom:
| Coelom | Pseudocoelom |
|---|---|
|
Coelom is a fluid filled true body cavity, present between the body wall and the digestive tract. |
Pseudocoel is a fluid-filled body cavity, present between the mesoderm and the endoderm. |
| Coelom is also known as eucoelom or true coelom. | Pseudocoelom is also known as false coelom. |
|
The animals with true coelom are known as coelomates and the group of animals is referred to as coelomata. |
Animals with a pseudocoel are known as pseudocoelomates and the group of animals is referred to as pseudocoelomata. |
|
Coelom or true body cavity is lined with peritoneum. | Pseudocoelom or false body cavity is partially lined with peritoneum. |
|
The coelom is developed by splitting of the mesoderm. |
The pseudocoelom is developed from the blastocoel of the embryo. |
|
In this case, the blood stream carries the nutrients. | In this case, the nutrient transportation occurs through osmosis and diffusion. |
| Coelom is present either in vertebrates or invertebrates. |
Pseudocoelom is present only in invertebrates. |
|
The organs present inside the coelom are Well-organized. |
The organs present inside the pseudocoelom are less-organized. |
|
Examples: Annelida, Arthropoda, Mollusca, Echinordermata, Hemichordata, Chordata, etc. |
Examples: Nematoda, Entoprocta, Rotifera, Acenthocephala, and Gastrotrica, etc. |
Functions/Significance of Coelom
- Coelom protects the internal organs from external mechanical shocks or trauma by surrounding them.
- It maintains the body shape and in earthworms and many other invertebrates, it functions as a hydrostatic skeleton to aid in locomotion.
- Coelom helps to eliminate excretory waste products from the body after metabolic process.
- Coelom is filled with coelomic fluid which act to separate the organs from the outer body.
- In some organisms, coelom helps to perform respiration by keeping the body wall moist.
- Coelom contains coelomocytes in the immune system of most invertebrates, acts as macrophage-like cells which are involved in phagocytosis, inflammation by enhancing immunity to the body and helps to destroy bacteria and other harmful organisms from the body.
- It provides flexibility to the body and helps to transport of gaseous substances and nutritive materials from one part of the body to the other.
- Coelomic cavity provides site for brooding of embryos, sperm and eggs during maturation.
- Coelomic fluid contains nutrients which are distributed to all parts of the body in such a manner of a circulatory system.
- Coelom acts as a reservoir for waste products and allows growth of internal organs.
Concluding Remarks
Coelom makes the partition to separate biological systems that perform various major functions. Acoelomates transport nutrients around the body through diffusion due to relatively small with less intricate body plan. In this case, diffusion is enough to perform transport function. The coelomates have complex body plan that need special transport system (circulatory system) to ship the nutrients around the body. In this case, coelom is useful to separate circulatory system from abdominal organs.