Final answer:
Animals with a cylindrical body tapered at both ends are roundworms (Nematoda), which are pseudocoelomates. They have a fluid-filled cavity that forms a hydrostatic skeleton, resulting in their round shape.
Step-by-step explanation:
The animals with a cylindrical body that is tapered at both ends are roundworms (Nematoda). Nematodes have a pseudocoelom, which is a fluid-filled body cavity that provides internal support, forming a hydrostatic skeleton. This physical feature contributes to their round shape, unlike the flat body plan of flatworms (Platyhelminthes).
Cnidarians, option A, display radial symmetry and are characterized by their cnidocytes, specialized stinging cells. Option C, Platyhelminthes, includes flatworms that do not have a cylindrical body but are instead often flattened dorsoventrally. Option D, Annelids, are segmented worms and have a more elongated body with visible segmentation rather than the uniform cylindrical shape of roundworms.