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Animals have germ layers, with sponges having one, cnidarians having two, and all other animals having three. Diploblastic animals have radial symmetry, while triploblastic animals have bilateral symmetry. The three germ layers are ectoderm, mesoderm, and endoderm. The mesoderm is an evolutionary innovation that allowed for complex and large animals with a coelom, which cushions organs and allows for a rudimentary hydrostatic skeleton.
The germ layers are the primary tissue layers in an animal, defined as clusters of cells. Sponges have only 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 symmetry.
The three germ layers are the ectoderm (outer layer), mesoderm (middle layer), and endoderm (inner layer). Cnidarians have only an ectoderm and an endoderm. Diploblastic animals are thought to have evolved very early in the history of multicellular life and may actually have been the first true multicellular organisms. Traditionally, scientists have thought that sponges evolved before the cnidarians, but more recent genetic analyzes have found that the 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 particularly evident during embryogenesis, when the embryo grows from a single fertilized egg into an organism ready to live on its own. In general, the ectoderm differentiates into the nervous system, epidermis, and outer part of the integumentary system (hair, scales, nails, sweat glands, etc.). The mesoderm differentiates into the wall of the intestine, which cushions and organizes the internal organs. The endoderm differentiates into the lining cells of most 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 reproduce effectively and to open the door to complex and large animals. Triploblastic animals can be up to 600 million years old. The key advantage 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. Additionally, the coelom allows for a rudimentary hydrostatic skeleton, which consists of muscles that pull against the wall of the coelom. This is not a hard skeleton but a soft “skeleton” possessed by all triploblastic invertebrate life.
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