Based on their body , flatworms and rotifers were not thought to be closely related. However, showed that they are a group.

Based on their body structure, flatworms and rotifers were not thought to be closely related. However, bilateral symmetry and body texture showed that they are a soft-bodied group.

Explanation:

Flatworm belongs to phylum Platyhelminthes and Rotifera belongs to phylum Rotifera. Their body structure may be different but both of them have a bilaterally symmetrical body.  Both are aquatic animals.  Flatworm is made up of ectoderm endoderm and mesoderm and has circular and longitudinal muscles. Rotifera is made up of skeletal and visceral muscles. So, they are soft-bodied animals.