Chapter 32: An Introduction to Animals Animals are a monophyletic group Animals are the only multicellular heterotrophs on the tree of life that ingest their food The origin and early evolution of animals was based on four aspects of the fundamental architecture, or body plan, of animals: 1. The origin and elaboration of tissues especially the tissues found in embryos 2. The origin and evolution of the nervous system and the subsequent evolution of a cephalized body one with a distinctive head region 3. The evolution of a fluidfilled body cavity; and 4. Variation in the events of early embryonic development The Origin and Diversification of Tissues: Dipoblasts = animals whose embryos have two types of tissue Tripoblasts = animals whose embryos have three types Ectoderm gives rise to skin and the nervous system Endoderm gives rise to the lining of the digestive tract Mesoderm gives rise to the circulatory system, muscle, and internal structures such as bone and most organs In general, the ectoderm produces the covering of the animal and the endoderm generates the digestive tract. Mesoderm gives rise to the tissues in between Two groups of animals have been recognized as dipoblasts: the Cnidaria (jellyfish, corals) and the Ctenphora. All other animals are tripoblastic. Nervous Systems, Body Symmetry, and Cephalization: Nerve net = nerve cells that are organized into a diffuse arrangement Ganglia =neurons clustered into masses Radial symmetry = have at least two planes of symmetry. Usually float in water or live attached to a substrate.