Diploblastic animals have radial symmetry, meaning that they can be divided into two similar halves in many different ways, while triploblastic animals have bilateral symmetry, which means that there is only one way to split them into similar halves. As a general simplification, it can be said that the ectoderm develops into the outer skin, and the endoderm eventually forms the digestive system, while the mesoderm — present only in triploblastic animals — develops into muscles and various internal organs. Diploblastic organisms are therefore very simple in that they essentially only have an outer skin, which may include a rudimentary nervous system, and a digestive tract. In the more complex triploblastic animals, such as mammals, things are more complicated. The brain, for example, develops from the ectoderm, along with the rest of the nervous system; some of the internal organs, such as the liver, pancreas, and various glands, arise out of the endoderm, along with the digestive system.
Triploblasts can be further divided in terms of body cavities. The simplest types, such asflatworms, do not have a cavity other than the digestive tract. Some other animals have a fluid-filled gap between the digestive tract and the mesoderm. The most advanced animals have a cavity that lies entirely within the mesoderm. This allows them to push food through the gut by means of muscle contractions.
The two main types of diploblastic animals are the Cnidaria and the Ctenophora. Cnidarians are mostly marine, but there are a few freshwater members of the group, which includes jellyfish, corals,sea pens, sea anemones, sea pansies, sea wasps, and sea fans. Ctenophores are a separate marine group, sometimes referred to as comb jellies. These simple animals lack true organ systems, but do have a cavity in which digestion of food takes place, and they may have nerves, sensory apparatus, and reproductive parts.
