• The word coelom describes the body cavity that is internal space. But there can be difference in coelom true coelom will always be fluid filled lying between outer bodywall and inner digestive tube. It arises between two layers of embryonic mesoderm and consist of most of the visceral organs.
  • Hence true coelom can be define as secondary body cavity, formed by splitting of mesoderm during during embryonic development and bounded by all sides definite coelomic epithelium or peritoneum.
  • The fluid which is present in coelomic cavity is colourless, the excretory organs open into it and the reproductive organs arises from its walls.
  • However all animal do not posses coelom such as porifera coelenterates Ctenophora etc and hence they are known as acoelomates.
  • On the basis of presence and absences of coelom the metazoans are divided into 3 major groups
  1. Acoelomates
  2. Pseudocoelomates
  3. Eucoelomates or coelomates
FIGURE DEPICTING :- Types of coelom


No body cavity or coelom is present embryonic mesoderm remains as solid layer. Space between endoderm and ectoderm that is space between gutwall and bodywall is filled with mesenchyme and muscle fibres. Examples are lower invertebrates such as porifera, coelentereta, ctenophore, platyhelminthes and Nemertinea.

Acoelomates lack a fluid-filled body cavity between the body wall and digestive tract. This can cause some serious disadvantages. Fluid compression is negligible, while the tissue surrounding the organs of these animals will compress. Therefore, acoelomate organs are not protected from crushing forces applied to the animal’s outer surface. The coelom can be used for diffusion of gases and metabolites etc. These creatures do not have this need, as the surface area to volume ratio is large enough to allow absorption of nutrients and gas exchange by diffusion alone, due to dorso-ventral flattening.



Body space is a pseudocoelom or false coelom. It is persistence of blastocoels enclose between  outer endoderm and inner ectoderm and not lined by mesoderm

Example :- Aschelminthes, ectoprocta and Acanthocephala.

A pseudocoelomate which are also refer as blastocoelomate is any invertebrate with a three-layered body and a pseudocoel. The coelom was apparently lost or reduced as a result of mutation in certain types of genes that affected early development. Thus, pseudocoelomates evolved from coelomate “Pseudocoelomate” is no longer considered a valid taxonomy group since it is not monophyletic. However, it is still used as a descriptive term.

Important characteristics:

  • lack a vascular blood system
  • diffusion and osmosis circulate nutrients and waste products throughout the body.
  • lack a skeleton
  • hydrostatic pressure gives the body a supportive framework that acts as a skeleton.
  • no segmentation
  • body wall
  • epidermis and muscle
  • often syncytial
  • usually covered by a secreted cuticle
  • most are microscopic
  • parasites of almost every form of life (although some are free living)
  • loss of larval stage in some

Pseudocoelomate phyla :- According to Brusca and Brusca, bilaterian pseudocoelomate phyla include:

  • Rotifera
  • Kinorhynca
  • Nematode
  • Nematomorpha
  • Acanthocephalan
  • Loricifera
FIGURE DEPICTING :- Pseudocoelom


Body space is a true coelom means cavity is enclosed by mesoderm on each side. From annelids to arthropods  lies in coelomate category.

There are three different ways through which coelom can arise during the mesoderm due to this coelomates are further divided into three categories


Coelom arises by splitting of endomesodermal which originated from blastoporal region of larva and extant between ectoderm and mesoderm. It is true coelom refer to as schizocoel.

Example :- most of the protostomia (annelids, arthropoda, mollusc, etc)

2. Mesenchymal coelomata

It is only seen in Phoronida where coelomal or a space is enclose by rearrangement of mesenchymal cells.

3. Enterocoelomata

Coelom arises in the form of mesodermal pouches from larval archenteron. After separation from endoderm, the pouches fused and expand until they touch the gut and the bodywall. Since the coelom arises from from larval enteron it is called enterocoel.

Examples :- Dueterostoma

Three type of eucoelom (A) Schizocoelomata (B) Mesenchymal coelomata (c) Enterocoelomata
Three type of eucoelom :- (A) Schizocoelomata (B) Mesenchymal coelomata (c) Enterocoelomata


  • Evolution of the coelom is of great significance in animals. It allow in progressive development of complex structures. It has direct contribution in development of excretory, reproductive and muscular system of complex bodies. In the evolution of metazoans  it is an evidence of increase in size and of diversity in structure an
  • No doubt the evolution of coelom made a major advanced ways of life met within coelomata phyla in comparison with acoelomates and pseudocoelomate phyla.
  • There are modification of coelom in different animals. In annelids coelom is divided by septa into chamber corresponding to somites. In mammals coelom is divided by mascualr diaphragm into thoracic and abdominal cavities with separate pericardial cavity around heart.
  • It is thought that probably coelom is a polyphyletic in origin. Clark postulate that coelom might have arisen independently a number of times in a various ways and also in various animal groups. As persistent blastocoels in pseudocoelomates, as an enterocoel in dueterostomes, as schizocoel in protostomes.

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  1. Jyoti yadav

    This is very useful,keep posting!!