Concept:This question tests knowledge of the main classes in the phylum Cnidaria and the ability to identify an organism from a different phylum.
Explanation:The phylum Cnidaria contains four well-recognised classes.
Anthozoa consists of sessile polyps such as sea anemones and corals.
Scyphozoa comprises the true jellyfishes, which are free-swimming medusae.
Cubozoa includes box jellyfish, which have a cube-shaped medusa stage.
Hydrozoa includes both polyp and medusa forms, such as Hydra and Obelia.
Turbellaria, however, is not a cnidarian class.
Turbellarians are free-living flatworms placed in the phylum Platyhelminthes.
Examples include planarians and other non-parasitic flatworms.
Since the question asks for the exception among the four classes of cnidarians, Turbellaria is the correct exception.
Answer:A. Turbellaria