Sponges are the simplest form of multi-cellular animals. They are very diverse and come in a large variety of colours, shapes and structural complexities. They range in heights of 1-200cm and in diameters of 1-150cm. They have partially differentiated tissues, and not true tissues. Sponges don’t have internal organs. They don’t have muscles, a nervous system, or a circulatory system. Their walls are lined with many small pores called ostia that allow water flow into the sponge.