Animals with bilateral symmetry produce a third layer in-between called mesoderm, making them triploblastic.
Germ layers will eventually give rise to all of an animal's or mammal's tissues and organs through a process called organogenesis.
The mesoderm germ layer forms in the embryos of animals and mammals more complex than cnidarians, making them triploblastic.
A theory suggests that this key innovation evolved hundreds of millions of years ago and led to the evolution of nearly all large, complex animals.
Organs formed inside a coelom can freely move, grow, and develop independently of the body wall while fluid cushions and protects them from shocks.