But new evidence from comparative biochemistry has shown that it is likely that flatworms had coelomate ancestors, and so they do not represent an ancestor of other protostomes in a simple progression from acoelomate to pseudocoelomate to coelomate. They come in both free-living and parasitic forms, and they are both marine and freshwater. Their flatness is a consequence of their acoelomate condition, because they lack any fluid to transport oxygen around to their cells, so all cells must be supplied with oxygen by diffusion, which is very inefficient and typically limits body thickness to no more than 1 mm. They provide us with a great example of cephalization. A type of flatworm called a blood fluke is responsible for a human disease, schistosomiasis .
Adventurous travelers can bring schistosomiasis back to the United States.
The fluke responsible for this disease spends part of its life in a freshwater snail and part of its life in humans. It typically enters humans through the feet, so walking barefoot in infested water is the way humans acquire this infection. A drug treatment is available, and the World Health Organization is working hard to make this more widely available. They are carnivorous, and their feeding style is marked by a proboscis that can inject venom into their prey. Nemerteans inject venom with a proboscis, but we also say a butterfly drinks nectar with a proboscis.