The animals that are symmetrical and triploblastic (composed of three cell layers) are the Platyhelminthes, or flatworms.It now seems likely that the first two of these groups are paraphyletic; in nature they contain some but not all descendants of a common ancestor. Molecular examinations suggest that the Platyhelminthes as a whole may be polyphyletic, having arisen as two independent groups from different ancestral groups. If this view is correct, most of the flatworms may belong to the Lophotrochozoa, a group within the animal kingdom that includes the phylus of molluscs and earthworms, while the rest belong near the base of animal diversity.
The Cestoda
An example of a flatworm in its natural habitat.
Cestoda is a class of parasitic flatworms, commonly called tapeworms. Its members live in the digestive organs of vertebrates as adults, and often in the bodies of various animals as parasites .There is no true circulatory system. Flatworms have no need for the circulatory systems because: their cells are close to the animal’s exterior surface, the coeloma (set of concentric tubes, with a gap between the gut and the outer tubes)is bathed in blood and oxygen and the digestive system performs these functions,rendering circulation not necessary.
The Trematoda
Trematoda is a class that contains two groups of parasitic worms, referred to as "flukes" most of the time. The lack of a need for a circulatory system is due to their cells being close enough to the animal’s surface coupled with the fact that the coeloma is bathed in blood and oxygen, the digestive system performs these functions, the spherical body shape allows diffusion (the spread of particles through random motion from regions of higher concentration to regions of lower concentration of materials) are simple and flat enough so diffusion is sufficient for these processes.
The Turbellaria
The Turbellaria are of the phylum Platyhelminthes (flatworms), and include all the sub-groups that are not exclusively parasitic. There are about 4,500 specific species. As in the trematoda and cestoda, their lack of circulatory systems means that they have to rely on diffusion for internal transport of metabolites. The process of diffusion is responsible for the spread of amino acids, electrolytes, and lymph fluids to maintain homeostasis.