A. colorful, marine worm whose body is flattened dorso-ventrally. swims
through the water using a wavelike motion.
phylum?

Phylum platyhelminthe

Explanation:

Although a few of the free living marine and terrestrial species are very beautiful, most species are not particularly attractive to the human mind. Their body is dorso-ventrally flattened. They exhibit bilateral symmetry.   They do not have a body cavity and are acoelomate.  The movement of most platyhelminthes is using a wavelike motion that is controlled by longitudinal, circular, and oblique layers of muscle as they contract and relax simultaneously.