Which animals are simple and what defines a complex creature?
Tissue complexity allows the development of complex organ systems, organisms containing multiple layers of tissue are capable of much more than those only able to make one or even none.
The phyla porifera, the sea sponges, contain between none and two layers of tissue depending on the species, and are so simple an organism there is great debate as to whether or not they can even be classed as animals.
Cnidarians comprise of jellyfish, sea anemones and hydras. They are more complex than sponges and containing stinging cells. Platyhelminthes break the barrier by containing a coelom, which allows complex organ growth, and from here-on up the phylogenetic tree, we just get more and more complicated.
From Crash Course: Biology on YouTube by Hank Green.