What is a 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 embryonic development. The inner layer of the coelom is lined by the mesodermal epithelium cells. Animals are classified based on the 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.
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Classification Based on Coelom
There are three such categories divided on the basis of the presence or absence of coelom. They are acoelomate, pseudocoelomate and coelomate.
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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 organised fashion.
Example - Roundworms, belonging to protostomes. Also, not all protostomes may be pseudocoelomates but all pseudocoelomates are protostomes.
Eucoelomate
They are also known as coelomates or 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 the peritoneum and it allows the organs to remain attached to each other so that they can be suspended in a particular order (in an organised manner) while still being able to move freely within the cavity. During the stages of development, the blastocoel in the gastrula gets replaced by the true coelom. Example - Annelida, Chordata, Arthropoda, etc.
Types of Coelomates
On the basis of embryonic development, Coelomates are further differentiated into Protostomes and Deuterostomes. In protostomes and deuterostomes, the process of coelom development is distinct. On the basis of formation, the coelom is divided into two types: Schizocoelom and Enterocoelom. Protostomes develop Schizocoelom and Deuterostomes develop Enterocoelom.
Protostomes: The establishment of the organism's mouth at the blastopore before its anus during embryonic development is considered to distinguish protostomes from other animals. Nematodes, arthropods, flatworms, annelids, and molluscs are well-known protostomes. Schizocoelomates is another name for them.
Deuterostomes: The word "deuterostome" means "second mouth." Deuterostomes are animals commonly defined by their anus the anus develops at the site of the blastopore before their mouth during embryonic development. Vertebrates, including humans, sea stars, and crinoids are examples of deuterostomes.
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Schizocoelom
In these organisms, the coelom originates from the splitting of the 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 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 for 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.
In this article we explained what coelom is. Also, we have provided you its types and classification. To read more such topics, visit our website.
FAQs on Coelom
1. What is a coelom in the context of animal biology?
A coelom is the main fluid-filled body cavity in multicellular animals, positioned between the digestive tract and the body wall. A key characteristic of a true coelom is that it is completely lined by tissue derived from the mesoderm, one of the three primary germ layers. This cavity houses and protects internal organs, allowing them to move and grow independently.
2. What are the three main types of animals based on the coelom?
Based on the nature of the body cavity, animals are classified into three principal groups as per the NCERT syllabus for Class 11:
- Coelomates (or Eucoelomates): Animals that possess a true coelom, a body cavity completely lined by mesoderm. Examples include annelids, molluscs, arthropods, and chordates.
- Pseudocoelomates: Animals that have a body cavity that is not fully lined by mesoderm. Instead, the mesoderm is present as scattered pouches. An example is the phylum Aschelminthes (roundworms).
- Acoelomates: Animals that completely lack a body cavity between the body wall and the digestive tract. The phylum Platyhelminthes (flatworms) is a key example.
3. What is the primary importance or function of having a coelom?
The coelom is functionally important for several reasons. It acts as a protective cushion for internal organs, shielding them from external shocks. The fluid within the coelom can also function as a hydrostatic skeleton in some invertebrates, aiding in locomotion. Furthermore, it allows for the independent growth and movement of internal organs and provides a medium for the transport of nutrients, gases, and waste products.
4. What is the key difference between a true coelom and a pseudocoelom?
The fundamental difference lies in their embryonic origin and lining. A true coelom is defined as a cavity that is entirely enclosed by mesodermal tissue on all sides. In contrast, a pseudocoelom (or false coelom) is a body cavity that is only partially lined by mesoderm; it is situated between the endoderm (gut lining) and the mesoderm, lacking a complete mesodermal sheath.
5. How does the coelom form during embryonic development?
The formation of a true coelom (coelomogenesis) occurs in two main ways, which is an important basis for classifying higher animals:
- Schizocoely: In this process, the coelom originates from the splitting of solid masses of mesodermal tissue. This is characteristic of protostomes like Annelida and Arthropoda.
- Enterocoely: Here, the coelom forms from pouches that pinch off from the archenteron (the embryonic gut). This method is found in deuterostomes, including Echinodermata and Chordata.
6. Why are humans considered coelomates?
Humans are considered coelomates because a true, mesoderm-lined coelom forms during our embryonic stage. While this single cavity does not persist in adults, it differentiates and subdivides to form the major body cavities that house our organs: the pleural cavity (lungs), the pericardial cavity (heart), and the peritoneal cavity (abdominal organs).
7. Why is the presence of a coelom a significant feature in animal classification?
The presence and type of coelom are significant evolutionary milestones and thus crucial for classification. It reflects an animal's structural complexity. The development of a coelom allowed for the evolution of more complex and efficient organ systems, greater body size, and improved locomotion. Classifying animals as acoelomate, pseudocoelomate, or coelomate helps establish evolutionary relationships between different phyla.
8. Are all triploblastic animals coelomates? Explain with examples.
No, not all triploblastic animals (animals with three germ layers) are coelomates. While being triploblastic is a prerequisite for forming a true coelom, it doesn't guarantee its presence. For example, Platyhelminthes (flatworms) are triploblastic but are acoelomate, as the space is filled with tissue. Similarly, Aschelminthes (roundworms) are triploblastic but are pseudocoelomates. Only phyla from Annelida onwards are both triploblastic and true coelomates.
