As mentioned previously, flukes are flatworms. Similar to the well-known Planarian, they are flat, bilaterally symmetrical, and typically have both male and female reproductive systems in the same animal. The notable exception to this is members of the Schistosoma species, which have flattened anterior regions, but the remainder of the body is rounded to accommodate life in a vein. Also, the schistosomes are diecious, with morphologically distinct males and females. The male is larger and has a "gynaecophoric canal" in which the longer, slimmer female resides.
All flukes have two suckers: an oral sucker and one on the ventral surface. They have primitive mouths, as well as primitive digestive, excretory, and nervous systems. The most highly developed system is the reproductive system.
An adult Fasciola hepatica is seen to the right. Most of the darkened area seen is the organism's reproductive organs. The second image is of a male and female Schistosoma mansoni. The thin female resides in the gynecophoral canal of the thicker male.
