Understanding natural selection and how traits are passed down. This provides the "why" behind animal diversity.
Does it have two layers (diploblastic) or three (triploblastic)? TEXT-BOOKS OF ANIMAL BIOLOGY A General Zoology ...
Start with Porifera (sponges) and move through Cnidarians, Platyhelminthes (flatworms), Mollusks, Annelids, and the massive group of Arthropods (insects/spiders). Understanding natural selection and how traits are passed