Acoelomates - Examples of Acoelomate Animals

What is a coelomic cavity?

Coelomic cavity is a fluid-filled space between the body wall and the alimentary canal. This space accommodates and protects internal organs. In some animals, the coelom is lined with mesothelium, and in some, like molluscs, it is undifferentiated. Based on the presence or absence of coelom, the animal kingdom has been grouped into three –

  1. Acoelomates (absence of coelom)
  2. Pseudocoelomates (false coelom)
  3. Coelomates (true coelom)

Acoelomates – Meaning

Acoelomates do not have a fluid-filled space between their body wall and alimentary canal. Thus their internal organs are not protected from external crushing forces. Acoelomates do have some disadvantages as they do not have coelom to diffuse metabolites and gases. But most acoelomates have a smaller body surface, and hence this use of coelom is negligible. Also, the absence of body cavities does not signify the absence of fluid currents in the body. Platyhelminthes and sponges are classic examples of acoelomates.

Acoelomate Animals

The body cavity of a true coelomate is lined with mesodermal cells. In acoelomates, the mesodermal cells attach to the sides of other cells, via an extracellular matrix. They are devoid of the body cavity. Thus, the organs formed are completely surrounded by the mesoderm. This type of body organisation is compact and is seen in lower animals. In most cases (not all), body cavities are seen in large-bodied animals rather than the smaller ones. The small larvae of large animals are often acoelomates. Later during the development process, the body cavity arises. There are some small animals that do have body cavities. Examples – priapulids, rotifers. Also, there are some large animals with no body cavities. Examples – parasitic flatworms, polyclads.

Examples of Acoelomates

  • Porifera – Sponges
  • Ctenophores – Comb jellies
  • Cnidarians – Sea anemone, Jellyfish
  • Platyhelminthes – Flatworms like tapeworm and flukes
  • Nemertea – Ribbon worms
  • Gnathostomulida – Jaw worms

Also Check: Animal Kingdom

Frequently Asked Questions

What is the difference between coelomate and acoelomate?

Animals that have their body cavities filled with fluid are called eucoelomates or coelomates. Their internal organs are suspended in the fluid cavity. Examples include arthropods, annelids, molluscs, hemichordates, echinoderms and chordates. Acoelomates do not have a fluid-filled coelomic cavity. Example – Platyhelminthes.

What are pseudocoelomates?

In some animals, the embryonic blastocoel acts as a fluid-filled cavity. This body cavity is partially lined with the mesodermal derivative cells. Here, the coelom is much reduced or lost. It is not considered a true coelom, and hence the name pseudocoelomates. Examples include rotifers, priapulids and nematodes (roundworms).

What is a blastocoel?

The fluid-filled segment that is formed in the blastocyst or blastula is called a blastocoel. Usually, the hollow sphere of blastomere cells surrounds the blastocoel. Blastocoel is a common feature in most vertebrates during embryonic development.

What are coelenterons and spongocoel?

The blastocoel (blastocyst cavity) of most acoelomates is occupied by mesoderm. Thus only coelenteron or spongocoel is present. The gastrovascular cavity in cnidarians is called coelenteron, and the paragastric cavity of sponges is called a spongocoel.

Explore: Difference between Coelomate and Acoelomate

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