Acoelomates: Definition, Coelom Cavity & Examples

Acoelomate is an animal that does not have a coelom or body fluid. Scientists frequently utilize a range of shared characteristics to categorize animals and determine which groups are most closely related. Although DNA data has significantly increased our understanding of this subject, it is still only one piece of the picture. One trait that has been used by scientists to distinguish between several phyla is the development of a bodily cavity. 

In a large animal, it is simple to identify a coelom, but in a microscopic animal, it could be difficult to do so. Because of the tiny size of the real body cavity, an acoelomate may appear to be a microscopic coelomate. Despite the fact that they are acoelomates, they have evolved several sophisticated traits such as organs, spines, and intricate intestines. Simple contrasting methods for surviving on our planet include having or not having a bodily cavity. The acoelomate body plan is straightforward but extremely efficient. 

Read more: Classification of animal kingdom

KeyTerms: Coelomates, Acoelomates, Pseudocoelomates, Endoderm, Mesoderm, Ectoderm, Triploblastic body, Embryonic cell layers, Germ cell layers, False body tissues, True body tissues.

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Acoelomates and Acoelomate animals

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Acoelomates are animals that lack a fluid-filled body located between their body wall and digestive tract.

• They also lack body cavities and these body cavities form in multicellular organisms.
• They have a triploblastic body, and their organs and tissues are developed from three tissue layers (also known as Embryonic cell layers, and germ cell layers).
• These three tissues are the Endoderm layer, mesoderm, and Ectoderm layer.
• The Endoderm layer is the innermost layer that forms the digestive tract, the Mesoderm layer is the middle layer and the organs are surrounded by it, the Ectoderm layer is the skin and is the outermost layer. 
• These tissues are lined up in order from inside to out.

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What is Coelom?

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A Coelom is a body cavity filled with fluid and is present in an animal. It is present between the body wall and the gut wall. The inner layer of the Coelom is surrounded by mesodermal cells. Animals are distinguished on the basis of the presence or absence of Coelom. An animal that possesses a Coelom is known as Coelomates.

Further, it can be categorized as coelom and acoelomate, pseudocoelomate, and coelomate. These are distinguished on the basis of the presence or absence of coelom in animals.

Acoelomate 

As mentioned earlier Acoelomates are animals that don't have fluid-filled body cavities. For example flatworms.

Pseudocoelomate

Pseudocoelomates have a false body cavity and this cavity is not lined by mesoderm; instead it is present between ectoderm and endoderm. For example aschelminthes.

Coelomates 

Animals that possess coeloms are called coelomates. For example arthropods, echinoderms, hemichordates, and chordates.

The diagram given below shows a Comparison between Acoelomate, Pseudocoelomate, and Coelomate.

Comparison of Coelom

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Coelomic cavity

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The coelomic cavity protects and envelopes the internal organs in animals. This Coelomic cavity is a fluid fill space located between the body wall and the alimentary canal.

Coelom 

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Examples of Acoelomate

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1) PHYLUM PLATYHELMINTHES

Phylum Platyhelminthes 

Phylum Platyhelminthes: Examples like tapeworms and fluke worms.

Phylum Platyhelminthes are

• Leaf-like structure
• Body is dorso-ventrally flattened 
• They are Triploblastic and show bilateral symmetry.
• They have three layers: Ectoderm, Mesoderm, and Endoderm. (Refer to the above figure)
• Mesoderm being the middle layer consists of Parenchyma cells, helps in transportation, and acts as a support. ‘
• Circulatory system and digestive system are completely absent in Flatworms.
• Anus is also absent so excretion takes place by an organ called a Flame cell.
• Nervous system consists of nerve fibers and ganglions.
• Reproductive system is well developed.

2) PHYLUM PORIFERA-SPONGES

Phylum Porifera 

Phylum Porifera: Example like sponges 

Phylum Porifera is an acoelomate animal that does not have a true body cavity. It is called sponges because it belongs to the phylum. 

Phylum Porifera sponges have

• An asymmetrical body plan.
• A water transport and canal system.
• They have tiny pores known as Ostia, in their outer walls through which the water enters into a central cavity known as  spongocoel, and it goes out through the osculum.
• The water flows in the same direction. (unidirectional) and this pathway helps in the excretion of waste products from animals, food gathering, and respiratory exchange.
• Collar cells which are known as choanocytes line the spongocoel and canals.

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Things to remember 

• Acoelomate is an animal that does not have a coelom or body fluid.
• They have a triploblastic body, and their organs and tissues are developed from three tissue layers.
• The Endoderm layer is the innermost layer that forms the digestive tract.
• The Mesoderm layer is the middle layer and the organs are surrounded by it.
• The Ectoderm layer is the skin and is the outermost layer. 
• Animals that possess coeloms are called coelomates.