They are very sensitive to any movement around them. If they feel any movement in the water or any new odours or smells, they become excited and will move about trying to discover what is making the change. They breathe through their skin like most worms. All leeches are 'hermaphrodites' like earthworms. That means that one leech has both male and female reproductive organs.
Some leeches then drop the eggs into the water.
Other leeches put the eggs in special little pockets that they stick to rocks. Still others carry the eggs in a capsule beneath their bodies. They carry this capsule for a few days after the young are hatched from the eggs. Tiny leeches hatch out and begin looking for victims. When the pond dries up during the summer, or turns cold in the winter, leeches burrow into the mud.