Coelom: Formation & Types

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  • 0:04 What Is a Coelom?
  • 1:36 Importance of a Coelom
  • 2:42 Coelom Formation
  • 5:12 Lesson Summary
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Lesson Transcript
Instructor: Jeremy Battista
A coelom is a body space found in many types of animals, including yourself. In this lesson, we'll look at the function of the coelom and two categorizations based on coelom formation: protostomes and deuterostomes.

What Is a Coelom?

A coelom is a fluid-filled space between an organism's internal organs and its body wall that acts as a cushion and a protective barrier. Basically, it prevents the organs from slamming against the outer wall of the body, protecting the internal organs from any sort of real massive trauma on a day-to-day basis. In complex organisms like us humans, this allows our organs to develop and grow. In simpler organisms, such as worm, the coelom actually acts most like a skeletal structure.

The coelom is a structure that has evolved over time and is only found in Kingdom Animalia. Animals with a coelom are called coelomates or sometimes eucoelomates, those with a true coelom. Among the coelomates, animal members are classified as protostomes or deuterostomes, depending on how the coelom forms during embryonic development. They also can be referred to as schizocoelomates or enterocoelomates respectively, again, because of how the coelom forms as an embryo.

Note, there are also pseudocoelomates, or those with a fake coelom (for example, roundworms) and acoelomates, those without a coelom (for example, flatworms). Their flat body types do not allow for such a distinct body feature. The pseudocoelom is partially lined with mesoderm and endoderm cells versus a coelom that is distinctly made of mesoderm cells. The pseudocoelom functions in much the same way as a coelom does; it's just a bit more primitive and made of different cells.

Importance of a Coelom


Let's first take a look at coelom's role in cushioning. The coelom is extremely important because it's a cushion for all of your internal organs. Think for a second about car accidents. When involved in one, your body continues traveling forward even after the car stops. You body is stopped by the seatbelt, but still your internal organs continue to press forward. By having a coelom, or internal body cavity, there is room for your organs to continue forward but enough cushioning inside that they can safely (in most cases) come to a stop without any major damage.

Having a coelom is critical because it allows your organs to grow, develop, and change over time. Without one, your organs would struggle to grow and change as there would be very limited room.

Skeletal Structure

Now let's take a look coelom's role in skeletal structure. In certain organisms that lack a skeleton, the coelom actually acts as a hydrostatic type of skeleton in that the coelom is pressurized and allows the organism, such as a worm, to move like they do. It gives it rigidity and structure that it otherwise wouldn't have.

Coelom Formation

The coelom is formed during the embryonic stage of development. There are two distinct ways in which the coelom can form, classifying coelomates as either a protostome or a deuterostome. In short, the major difference between the two types is in the way cleavage divisions occur during development of the organism. In both protostomes and deuterostomes, the coelom forms in an area in conjunction with the mesoderm. Formation differs in that protostomes form a coelom when the mesoderm splits, whereas the deuterostomes form a coelom when mesoderm cells combine, resulting in the body cavity.

This formation occurs during the gastrula, or three-layered structure stage of development. In both protostomes and deuterostomes we see this blind pouch, or archenteron, form as the mesoderm bends inward. This is where the two types begin to differ, which will be explained now.

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