Final answer:
The Bilateria are classified into three groups based on body cavity type: acoelomates without a body cavity, pseudocoelomates with a semi-lined body cavity, and coelomates with a fully mesoderm-lined body cavity.
Step-by-step explanation:
The Bilateria may be classified on the basis of the type of body cavity they have into acoelomates, pseudocoelomates, and coelomates. None of the other options are correct.
Acoelomates lack a body cavity altogether, having solid bodies without a fluid-filled cavity between the gut and the body wall. An example of acoelomates are the flatworms.
Pseudocoelomates have a body cavity known as a pseudocoelom, which is partially lined with mesoderm. This coelom forms between the mesoderm and the endoderm. The roundworms are examples of pseudocoelomates.
Coelomates (or eucoelomates) have a true coelom that arises entirely within the mesoderm and is completely lined with mesodermal tissue. This coelom serves several vital functions, such as protecting internal organs and aiding in motility. Animals such as earthworms, snails, insects, starfish, and vertebrates are eucoelomates.