The Platyhelminthes are traditionally called as Flatworms; they are bilateral symmetry, which contributes the fact that they are Parazoan (Sponge). They are un-segmented, triploblastic (three germ layers) organism. These are the first organisms to exhibit three distinct cell layers, the ectoderm, mesoderm, and endoderm. Platyhelminthes have an acoelomate body cavity; this means that these organisms are lacking a coelom.
The body is quite abnormal, hydrostatic skeleton, flattened and no segmentation and also there is no-cavity and no respiratory systems are present, they exhibit cephalization (most sense organs centralized in the head). The Platyhelminthes have different type of the nervous system which has two nerve cords runs length of the body and concentrated in head called ganglia (brain) to which they respond to light and eyespot and these system is connected to several muscle layers so they can change the shape and move.
The circulation in the Phylum Platyhelminthes occurs due to diffusion. Flatworms have only one digestive opening that branch to all parts of the body. Platyhelminthes brings the food in to gastro vascular cavity which is highly branched through Pharynx; enzymes digest food in cavity (extracellular digestion) as the nutrients absorbed in to all cells directly from gastro vascular cavity so no circulatory system needed for them. Depending on the type of organism in the phylum, mobility differs. Turbellarians have a muscular pharynx to move, while parasitic worms rely on the host circulation. In Platyhelminthes sexual reproduction occurs through fragmentation or by transverse fission, since flatworms are hermaphrodites, sexual reproduction occurs through the exchange of the sperm, fertilization occurs internally and proceeds through a larval stage. When flatworms encounter each other they engage in a 60 minute dance during which they repeatedly strike at each other, both trying to inject their sperm under the skin of other worm.
The parasitic Platyhelminthes (Neodermata) controls bodily fluids which help them to live better; they are free living in marines, fresh water, in terrestrial organisms or in the parasitic organisms. The Platyhelminthes has four classes of flatworms each of the flatworms having a unique morphology, and life style including turbellaria: Free living, Nonparasitic lives in water or can live on the ground, Manogenea and Trematoda consists of parasitic flatworms lives inside or surface of other animals and Cestoda consists of tapeworms are also parasites lives in host intestine and absorb the nutrients because of these they dint have locomotive system… The tapeworms that have long flat bodies, the scolex is a holdfast head portion with 4 suckers and hooks. Tapeworms lack a digestive system, because they adsorb nutrients through the length of their body. They can consequently grow up to 60 feet in humans and 100 feet in whales.