The following animals is an invertebrate except

Invertebrates are animals that do not have a backbone or spinal column. They make up the vast majority of the animal kingdom, with about 97% of all animal species being invertebrates. One of the options listed is not an invertebrate, and that is "chordate." Chordates are a group of animals that have a notochord, which is a flexible rod-like structure that runs along the length of their body. They also have a dorsal nerve cord and pharyngeal gill slits at some point in their life cycle. Examples of chordates include fish, birds, mammals, and reptiles. The other options listed are all invertebrates. Flatworms and roundworms are both types of worms that do not have a backbone. Protozoans are single-celled organisms that do not have a backbone or any other type of skeletal structure. Therefore, the correct answer is "chordate," which is not an invertebrate because it has a notochord and other characteristics that are not found in invertebrates. The other options listed are all invertebrates because they do not have a backbone or spinal column.