Acoelomates: which statement best describes them?

Acoelomates: which statement best describes them?

• A. They lack a true coelom
• B. They have a true coelom but no mesoderm
• C. They have a hydroskeleton
• D. They have a complete mesoderm around the coelom

Answer: (A)

Acoelomates lack a true coelom, meaning they do not have a fluid-filled body cavity entirely lined by mesoderm. In these animals, the mesoderm is not organized into a hollow cavity; it forms a solid mass of cells called parenchyma that fills the space between the gut and the body wall. Because there isn’t a closed, mesoderm-lined cavity, their internal support and organization come from this solid tissue and from body wall and muscle layers rather than from a coelomic cavity. This is why the idea that they have a true coelom is not accurate, and why the other statements don’t fit: a true coelom cannot exist without mesoderm forming a lining around it, a true coelom would imply a distinct cavity, and a hydrostatic skeleton typically relies on a fluid-filled cavity, which acoelomates lack. For reference, flatworms are classic examples of this group.

Acoelomates lack a true coelom, meaning they do not have a fluid-filled body cavity entirely lined by mesoderm. In these animals, the mesoderm is not organized into a hollow cavity; it forms a solid mass of cells called parenchyma that fills the space between the gut and the body wall. Because there isn’t a closed, mesoderm-lined cavity, their internal support and organization come from this solid tissue and from body wall and muscle layers rather than from a coelomic cavity. This is why the idea that they have a true coelom is not accurate, and why the other statements don’t fit: a true coelom cannot exist without mesoderm forming a lining around it, a true coelom would imply a distinct cavity, and a hydrostatic skeleton typically relies on a fluid-filled cavity, which acoelomates lack. For reference, flatworms are classic examples of this group.