What are Germ Layers?
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
