In these animals oxygen can diffuse through the body surface and reach easily to energy producing organelles. Similarly, the waste products can rapidly move from the body by simple diffusion.
Multi-cellular organisms have a proper transport system to carry substances from one part of the body to another. The system involved in the transport of various substances within the body of an animal is called the Circulatory system.The circulatory system transport gases like O2, CO2 etc. nutrients, wastes, hormones and defence proteins
