Monogenea are microscopic parasitic flatworms that are mainly found on epidermis of humans or gills . They are rarely longer than about two centimeters. A few specific species infect certain marine animals and are larger than those found on fresh water hosts. Monogeneans lack respiratory, skeletal and circulatory systems and have no or weakly developed suckers. Monogenea attach to their hosts using hooks and/or clamps. They move by shortening and elongating their bodies by contracting their muscles.