Solid worms, such as flatworms, lack any internal body cavity or coelome, and hence, they are acoelomates.
Coelome:
It is the cavity between the alimentary canal the body wall of the animal. In this many organs such heart and lungs are present.
For example- Mollusc, mammals, annelids.
Some animals does not have any body cavity so, they are known as acoelomates.
For example - Flatworm.
Therefore, solid worms, such as flatworms, lack any internal body cavity or coelome, and hence, they are acoelomates.
To know more about Coelome:
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