The phylum Platyhelminthes includes all the species of flatworms. They are what most biologists consider to be the simplest and probably the most primitive bilaterally symmetrical animals. They are of great scientific interest because of their simplicity and because among them are found the most insidious, disease-causing organisms known to man, the schistosomes. These parasitic flatworms cause misery and death to tens of millions of human beings around the world each year.
As their name implies, almost all members of this phylum are extremely flat, ranging in size from microscopic lengths to several feet. Like the coelenterates, their bodies are composed of three layers of cells; ectoderm, mesoderm, and endoderm, but unlike that of the coelenterates, the mesoderm is well developed and contains many cells, filling solidly the space between the gut and the body wall. The fact that there is no body cavity between the gut and the body wall is of considerable evolutionary significance.
Their organs for reproduction and excretion, as well as muscles, are also very well developed. They have also a simple central nervous system with a primitive brain. However, flatworms have no skeleton and a no special organs for circulation or respiration (such as heart and lungs). Nutrients and oxygen are absorbed and distributed by diffusion from cell to cell.
There are about 7,000 species of flatworms divided into three classes; the free-living Turbellaria and the parasitic Trematoda and Cestoda. In adapting to a specialized life, the parasitic classes have lost many of the features that typify the phylum and, therefore, the Turbellaria hold the most evolutionary interest.