Flatworm

Platyhelminthes are bilaterally symmetrical animals: their left and right sides are mirror images of each other. This also implies that they have distinct top and bottom surfaces and distinct head and tail ends. Like other bilaterians, they have three main cell layers (endoderm, mesoderm, and ectoderm), while the radially symmetrical cnidarians and ctenophores (comb jellies) have only two cell layers. Beyond that, they are "defined more by what they do not have than by any particular series of specializations." Unlike most other bilaterians, Platyhelminthes have no internal body cavity, so are described as acoelomates, although the absence of a coelom also occurs in other bilaterians such as gnathostomulids, gastrotrichs, xenacoelomorphs, cycliophorans, entoprocts and the parasitic mesozoans.

Cestodes (tapeworms) and digeneans (flukes) cause diseases in humans and their livestock, whilst monogeneans can cause serious losses of stocks in fish farms. Schistosomiasis, also known as bilharzia or snail fever, is the second-most devastating parasitic disease in tropical countries, behind malaria. The Carter Center has estimated that 200 million people in 74 countries are infected with the disease, half of whom live in Africa. The condition has a low mortality rate, but usually presents as a chronic illness that can damage internal organs. It can impair the growth and cognitive development of children and increase the risk of bladder cancer in adults. The disease is caused by several flukes of the genus Schistosoma, which can bore through human skin; those most at risk use infected bodies of water for recreation or laundry.