VERY small organisms such as Amoeba, Protococcus, Paramecium, Hydra, Spirogyra and Mucor have by virtue of their smallness a very large surface area compared with their volume.
Being so tiny, their material requirements and their waste products cannot be large in quantity, and the area they expose to their environment is great enough compared with their volume to enable them to obtain and get rid of all these materials easily. Even the sphere affords sufficient surface area in the tiniest forms. An increase in area is generally affected by elongation, by flattening, or by becoming cylindrical. We might include here also the cylindrical Nematodes— threadworms, such as Ascaris.
In higher forms of life which are larger, the surface is no longer sufficiently big to carry out these functions, for the cells forming the inner parts of the organism would starve and also become poisoned with their own waste products if they could not be removed rapidly enough through the surrounding cells nearer the outside. Moreover, cells in higher forms are specialized into tissues and organs to perform particular tasks and are not generalized like the cells of simpler organisms which carry out all the functions. These functions we find to be carried out in one of two ways :—
By the presence of a diffuse type of alimentary canal and excretory system, admirably exhibited in the liver fluke. The tapeworm, another endoparasite belonging to the same phylum, is flat enough to absorb sufficient nourishment through its surface, and is sluggish enough for a simple, much-branched excretory system to suffice for its needs.
By means of a series of tubes through which all materials can be transported from one part of the organism to another—such tubes form the blood system in animals and the xylem vessels and phloem in higher plants.