A leech is a blood sucking animal who lives off blood. I should find one lurking under a stone waiting for a feed of blood to pass by. A leech can't swim, it can only move by sticking one end on to something like a rubber dart. However, the front sucker has jaws which it can sink into any passing meal such as fish or snails. When it is not eating it presses itself flat against the stone so that nothing can prise it off.
Leeches feed on blood.
They loop the loop.