Pseudocoelomate: Definition and Examples

Pseudocoelomate Definition

A Pseudocoelomate is a group of organisms with a three-layered body that has a fluid-filled body cavity that is not derived from the mesoderm as it present in the true coelom body cavity. However, the body cavity is derived from the blastocoel so which is also known as the blastocoelomate.

In pseudocoelomate, a perivisceral cavity is present which is not a true coelom. A true coelom which lined with the peritoneum that separates the fluid from the body cavity. The organs receive their nutrient and oxygen from the fluid in the body cavity.

It derived from the embryonic blastocoel rather than the secondary cavity within the mesoderm. Blastocoel is a space between the gut and the mesodermal and ectodermal components of the body wall, it is not lined with peritoneum.

The presence of the blastocoel in the embryo is a universal condition in all metazoan. In most of the metazoans, the mesoderm becomes the lining of the body cavity which creating the true coelom. Some of the pseudocoelomates represent the primitive form of coelomates, and their ancestors don’t have a true coelom. Some of the other organisms have lost the peritoneum and have regressed to the pseudocoelomate condition.

Some of the larval forms of coelomates are started as pseudocoelomates. A pseudocoelomate is often a small animal, which mostly depends on diffusion to distribute oxygen and nutrients to its cell. They don’t have a circulatory system, however, they have an open circulatory system known as hemolymph which circulates a blood-like substance within the body cavities of these animals. Due to this, the cavity is referred to as the hemocoel and the organism as a homocoelomate.

Examples of Pseudocoelomate


Rotifers are extremely small multicellular organisms that exist by adhering to a substrate and filter-feeding from the water. Rotifers have a distinctive head region with hundreds of cilia that work together to create streams of water that direct food debris into the mouth. Like other metazoa, the rotifers have three different tissues or are triploblastic.

The embryo of a rotifer forms a blastula, similar to a human embryo. The blastocoel in the embryo develops into a cavity that is not covered by the peritoneum. The internal organs are immersed in liquid and oxygen is diffused directly through the outer layers of the small animal. This is the typical form of a pseudocoelomate and is beneficial for the small organism as it needs a separate circulatory system to circulate oxygen. These tiny organisms can be seen below.



Like the rotifers, the nematodes are small pseudocoelomates with a modified body cavity. While the pseudocoelomate state is sometimes thought to be ancestral, many organisms such as the nematodes have advanced body parts. Nematodes reproduce sexually, produce games, and have advanced organs for reproduction and foraging, as shown in the image below. These organisms can live freely or be parasitic in soil or water. Because of their advanced reproductive systems, these tiny organisms have adapted to nearly every environment on the planet.

C elegans male