Pseudocoelomates: which feature describes them?

Pseudocoelomates: which feature describes them?

• A. Partial mesoderm surrounding the coelom and a hydroskeleton for support
• B. Full mesoderm surrounding a true coelom
• C. No mesoderm surrounding the coelom
• D. External skeleton

Answer: (A)

Pseudocoelomates have a pseudocoel, a fluid-filled cavity that lies between the endoderm and the outer mesoderm. The mesoderm surrounds only part of this cavity, so the cavity is not fully lined by mesoderm—hence the term “false coelom.” That partial mesoderm arrangement combined with a fluid-filled cavity creates a hydrostatic skeleton: muscles contract against the fluid, providing support and enabling movement. This description matches the idea of partial mesoderm around the coelom and a hydrostatic skeleton for support. In contrast, a true coelomate has a cavity completely lined by mesoderm, an acoelomate has no cavity lined by mesoderm, and an external skeleton is not the defining feature of pseudocoelomates.

Pseudocoelomates have a pseudocoel, a fluid-filled cavity that lies between the endoderm and the outer mesoderm. The mesoderm surrounds only part of this cavity, so the cavity is not fully lined by mesoderm—hence the term “false coelom.” That partial mesoderm arrangement combined with a fluid-filled cavity creates a hydrostatic skeleton: muscles contract against the fluid, providing support and enabling movement.

This description matches the idea of partial mesoderm around the coelom and a hydrostatic skeleton for support. In contrast, a true coelomate has a cavity completely lined by mesoderm, an acoelomate has no cavity lined by mesoderm, and an external skeleton is not the defining feature of pseudocoelomates.