What are common mistakes students make with animals?
Since we are mammals, we tend to focus on mammals or the very least on vertebrates. Due to their complexity, we are naturally more interested in them.
We often forget that sponges, cnidarians, worms, and Echinodermata are animals.
We often forget that not all animals have bilateral symmetry, heads (as a result of cephalization), blood, hearts, mouths or anuses.
Instead of studying animals as organisms in separate categories, try to view them in a phylogenetic manner.
For example, the development of a anterior or head region occurred during the evolution of worms when certain sensory organs were concentrated in a cephalization area.
See each major group of animals as a branching point in the tree of life allowed animals to adopt new adaptations that gave them an edge in survivial and reproduction of their species.