Pseudocoelomate Definition

A pseudocoelomate is an organism with body cavity that is not derived from the mesoderm, as in a true coelom, or body cavity. A pseudocoelomate is also known as a blastocoelomate, as the body cavity is derived from the blastocoel, or cavity within the embryo. A true coelom is lined with a peritoneum which serves to separate the fluid from the body cavity. In a pseudocoelomate, the body fluids bath the organs, and receive their nutrients and oxygen from the fluid in the cavity.

The presence of the blastocoel in the embryo is a condition universal to all metazoan. In most metazoans, the mesoderm becomes the lining of the body cavity, creating the true coelom. Some pseudocoelomates represent the primitive form of coelomates, and their ancestors never had a true coelom. Other organisms have lost the peritoneum and have regressed to the pseudocoelomate condition. The larval form of some coelomates start as pseudocoelomates. A pseudocoelomate is often a small animal, which relies mostly on diffusion to distribute oxygen and nutrients to its cells. These organisms typically have no circulatory system, or an open circulatory system which circulates a blood-like substance known as hemolymph within the body cavities of these animals. Because of this, the cavity is referred to as the hemocoel and the organism as a hemocoelomate.

Examples of Pseudocoelomate


Rotifers are extremely small multicellular organisms which exist by attaching to a substrate and filter feeding out of the water. Rotifers have a characteristic head region with hundreds of cilia, which work in unison to create currents of water that direct food particles to the mouth. Like other metazoans, the rotifers possess three distinct tissues, or are triploblastic. The embryo of a rotifer forms a blastula, much like a human embryo. The blastocoel inside the embryo develops into a cavity which is not covered in peritoneum. The internal organs are bathed in fluid, and oxygen in diffused directly through the outer layers of the small animal. This is the typical form of a pseudocoelomate, and is advantageous for the small organism because it does need a separate circulatory system to circulate oxygen. These tiny organisms can be seen below.



Like the rotifers, the nematodes are small pseudocoelomates which have a modified body cavity. While the pseudocoelomate condition is sometimes thought of as ancestral, many organisms like the nematodes have advanced body parts. Nematodes reproduce sexual, produce games, and have advanced organs for reproduction and finding food, as seen in the following image. These organisms can be free-living in the soil or water, or can be parasitic. Because of their advanced reproductive systems, these small organisms have adapted to nearly every environment on the planet.

C elegans male

  • Acoelomate – An organism which lacks a coelom completely, either due to lack of body tissues or a derived state where the body cavity has become filled in with cells.
  • Eucoelomate – Also called a coelomate, this is the condition of a “true” body cavity which is lined with peritoneum and mesentery and separates the organs from the cavity and body.
  • Blastocoel – The hollow cavity in the blastula, which eventually develops into the blastocoelom or gets filled in by cells.


1. An population of organisms has evolved throughout time. The ancestor of the organism contained only two tissue layers, and no body cavity in between them. The population evolved a third cell layer, and eventually a full coelom. The population evolved to be much smaller, and the coelom was lost. Which of the following best describes all the states of this population?
A. Pseudocoelomate
B. Eucoelomate, Acoelomate, Pseudocoelomate
C. Acoelomate, Eucoelomate

Answer to Question #1
B is correct. This population of organisms has exhibited all forms of body cavities throughout evolutionary time. It is theorized that this is probably what has happened in many groups of animals. The differing forms of body cavity typically are driven by evolutionary forces that select for various attributes of each form. Some small organisms are coelomates, while others are pseudocoelomates. The processes that drive these different lines of evolution are not fully understood.

2. Many pseudocoelomates are able to move their bodies in complex ways to move through the environment. How can this be accomplished, seeing that no pseudocoelomates have any sort of skeletal system?
A. They are pushed by currents
B. The fluid in the body cavity is manipulated
C. The microtubules in their cells are stronger

Answer to Question #2
B is correct. The movement of animal with water pressure is called hydrostatic movement, and the fluid can even be considered a type of skeleton. If you’ve ever squeezed a water balloon, you know that the fluid must go somewhere, and pushed into the other half of the balloon. The same concept helps move pseudocoelomate animals. They usually have small muscles which exert a pressure on their body cavity. The fluid pushes and expands the cavity, allowing the organisms to move and stretch.

3. An embryo develops into a blastula, which has a blastocoel. This cavity develops into a full cavity in the larva, and is filled with fluid. As the larva finds a place to settle down, the organism becomes sessile, and the cavity fills with cells. There is no longer a functional cavity. Which term describes the final condition of the organism?
A. Pseudocoelomate
B. Coelomate
C. Acoelomate

Answer to Question #3
C is correct. The adult organism would be considered an acoelomate, because it lacks a body cavity. The larval form would be considered a pseudocoelomate. A true coelomate is not seen in this organism, as the cavity is never lined and derived from the mesoderm. Many organisms show patterns of changing their coelom as they develop.