They have survived every catastrophe and every mass extinction event that nature has thrown at them. And by being the little, filter-feeding, water-cleaning creatures that they are, sponges may have saved the world.

It's finally time to start diving into individual animal phyla! First up is Porifera. This includes all the sponges. These are funky looking organisms, almost none of which exhibit any kind of symmetry, nor do they possess any tissues or...