Summary
Acoelomate, pseudocoelomate and coelomate animals differ in how the fluid-filled cavity around organs is formed.
In depth
Coelom: fluid-filled cavity lined by mesoderm (annelids, vertebrates). Pseudocoelom: only partially lined (nematodes). Acoelomate: solid body (flatworms). A coelom allows organs to grow independently and acts as a hydrostatic skeleton.
The type of cavity is used to classify animals. A coelom provides space for organs and hydrostatic support.
Key idea: coelom evolution allowed larger, more complex bodies.
A true coelom, lined with mesoderm on both sides, allows organs to move and grow independently of the body wall, cushions organs, and acts as a hydrostatic skeleton in annelids. Coeloms form by schizocoely (splitting of mesoderm, protostomes) or enterocoely (pouches from the gut, deuterostomes). In humans, the coelom becomes the pleural, pericardial and peritoneal cavities.
Acoelomate flatworms rely on diffusion and have no body cavity, and pseudocoelomates (nematodes, rotifers) have a cavity not fully lined by mesoderm. Some animals have lost the coelom secondarily, and arthropods and molluscs have a reduced coelom with a haemocoel (blood-filled space).
Example: peritonitis is an inflammation of the lining of the abdominal coelomic cavity.
Frequently asked questions
What is Body Cavities?
Acoelomate, pseudocoelomate and coelomate animals differ in how the fluid-filled cavity around organs is formed.
Which area of biology covers Body Cavities, and how advanced is it?
Body Cavities is one of the 53 topics in Zoology & Animal Diversity. It is rated Intermediate, so it suits learners who already know the core vocabulary. Related topics in this area include Germ Layers, Protostomes and Deuterostomes, Animal Kingdom.
Can you give an example of Body Cavities?
Earthworm segments each have their own coelomic chamber.
What should I learn before Body Cavities?
Work through the Beginner topics in Zoology & Animal Diversity first, such as Animal Kingdom, Vertebrates and Invertebrates, Sponges, then return to Body Cavities. Intermediate topics assume the Beginner vocabulary.
What topics are related to Body Cavities?
The closest topics to Body Cavities are Germ Layers, Protostomes and Deuterostomes, Animal Kingdom, Animal Skeletons. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Body Cavities?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Body Cavities. To practise the whole of Zoology & Animal Diversity, use the Zoology & Animal Diversity quiz.
How is Body Cavities different from Germ Layers?
Body Cavities: Acoelomate, pseudocoelomate and coelomate animals differ in how the fluid-filled cavity around organs is formed. Germ Layers: Ectoderm, mesoderm and endoderm give rise to all animal tissues; simple animals have only two. Read both side by side and note what each one explains.
How should I study Body Cavities step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Germ Layers and explain the difference aloud. Finally, take the Zoology & Animal Diversity quiz and come back to this page for any question you miss.