Germ layers are the primary tissue layers in an animal, defined as groups of cells. Sponges have just one germ layer, cnidarians (jellyfish and relatives) have two, while all other animals have three. Animals with two germ layers are called diploblastic while those with three are called triploblastic. Most triploblasts have bilateral symmetry, while diploblasts have radial symmetry. Most sponges lack any symmetry.
The three germ layers are the ectoderm (outer layer), mesoderm (middle layer), and endoderm (inside layer). Cnidarians only have an ectoderm and an endoderm. It is thought that diploblastic animals evolved very early in the history of multicellular life, and may have actually been the first true multicellular organisms. Traditionally, scientists have thought that sponges evolved before cnidarians, but more recent genetic analysis has found that cnidarians are actually the oldest, and sponges are in fact a secondarily simplified phylum, probably evolving from diploblasts.
The division of the germ layers is especially evident during embryogenesis, when the embryo grows from a single fertilized egg into an organism ready to live on its own. Generally speaking, the ectoderm differentiates into the nervous system, epidermis, and the outer part of the integument system (hair, scales, nails, sweat glands, etc). The mesoderm differentiates into the wall of the gut, which cushions and organizes the internal organs. The endoderm differentiates into the lining cells of most of the internal organs, including the entire digestive system. Together the germ layers make up the organism.
The mesoderm is the great evolutionary innovation that allowed our triploblastic ancestors to effectively reproduce and open the door for complex, large animals. Triploblastic animals may be as much as 600 million years old. The key benefit of the mesoderm, unique to triploblasts, is the coelom, or body cavity, which has numerous useful functions. By cushioning the internal organs from external pressure, the organism is much less fragile. The coelom allows organs to grow independently of the body wall and keeps them organized. Also, the coelom allows a rudimentary hydrostatic skeleton, which consists of muscles pulling against the coelom wall. This is not a hard skeleton but a soft "skeleton" possessed by all triploblastic invertebrate life.