The following animals are invertebrate except

Analyze the question: The question asks which option is NOT an invertebrate. "Except" means we are looking for the odd one out, the group that has a backbone (vertebral column) or is related to vertebrates.

Explain the concept:

• Invertebrates are animals without a backbone. They include protozoans, sponges, cnidarians, flatworms, roundworms, annelids, molluscs, and arthropods.
• Chordates are animals that have a notochord at some stage of their life. The vast majority of them (fishes, amphibians, reptiles, birds, mammals) are vertebrates, with the notochord replaced by a backbone in the adult.

Step-by-step solution:

1. A. Flatworms (Phylum Platyhelminthes, e.g. tapeworm, liver fluke): no backbone, so invertebrate.
2. B. Roundworms (Phylum Nematoda, e.g. Ascaris, hookworm): no backbone, so invertebrate.
3. C. Chordate (Phylum Chordata): includes the vertebrates, which have a backbone. This is the odd one out.
4. D. Protozoan (Kingdom Protista, e.g. Amoeba, Paramecium): single-celled, no backbone, so invertebrate.

Note on accuracy: Strictly speaking, Phylum Chordata also contains a few small invertebrate chordates (like lancelets and sea squirts), which have a notochord but no true backbone. However, the question is testing the main idea that chordates are the group containing vertebrates, and the other three options are clearly invertebrate groups.

Answer: C. Chordate

Exam tip: Remember "Chordates = Backbone (mostly)". When you see "except" in a classification question, find the group that differs from the others. Here, three groups are worms or single-celled organisms, and only one is linked to animals with backbones.