All animals are multicellular, eukaryotic heterotrophs that are motile at some stage in their lifetime and are able to reproduce sexually.
Porifera, Cnidaria, Platyhelminthes, Annelida, Mollusca, Arthropoda, and Chordata make up the seven major phyla of the animal kingdom.
By observing the form of a structure, we can predict its function.
Some animals are radially symmetric, which means their bodies are symmetrical when divided along a central axis. Some others are bilaterally symmetric, therefore have mirror-image halves along a single axis. Porifera have no symmetry, thus they are asymmetrical.
Animal species have specialized to function under specific conditions in different niches. This niche specialization has caused the animals to have diverse traits.