A key from one telling feature

Each non-chordate phylum has a feature that gives it away. You will finish a key that names the phylum from one feature, then use it on a list of animals.

The idea. A dictionary can hold a key. Each telling feature is a key of the dictionary, and the phylum is its value.

Your tasks.

1. Add two entries to key: "comb plates" for "Ctenophora", and "flame cells" for "Platyhelminthes".
2. In phylum_of, return the phylum for a feature, or "unknown" if the feature is not in the key.
3. Set cnidarians to a list of the names of the animals that are cnidarians, in the order of animals.