Germ layers

Abstract The idea that the embryo and the adult animal can be divided into a small number of distinct cell populations or germ layers is known as the germ layer theory. The first of the germ layers is the ectoderm, which gives rise to the outer surfaces of the animal. The second is the endoderm, which gives rise to the inner surfaces. These two germ layers mostly generate epithelial tissues. The third germ layer is the mesoderm, which gives rise to connective tissues and support structures. The outer surface of the animal is known as the integument. It forms the boundary between the organism and the outside world, but also has many important roles in thermoregulation, in communication with other organisms and in sensing the environment. The inner surface is the digestive system. It is divided into a number of functional regions that are responsible for the different stages of digestion, from initial breakdown of the food to the absorption of digested nutrients and removal of undigested material. The structure of the digestive system is strongly dependent on the animal’s food source and mode of feeding. Animals can feed on suspended matter in the water column or can eat larger food items, be they other animals or plants.