ABSTRACT
Except in very small animals (e.g. flatworms) or those with very low metabolic rates (e.g. jellyfishes), simple diffusion from the surface of the body cannot distribute gases and nutrients, metabolites and excretory products, and hormones and defense components to the relevant cells. Most animals contain a transport fluid (blood or hemolymph): localized stirring of the fluid by bodily movement is enough to facilitate transport in simple animals; some form of mechanical pumping must be used in larger, more complex forms.
