3 main classes of leeches (no pronunciation help here, though gnatbobdellida seems like a good password and makes me think of mr. bob dobalina):
Gnatbobdellida - have jaws, bite their hosts with their teeth, and secrete hirudin, an anticoagulent.
Rhyncobdellida - no jaws, but they insert a proboscis into the host, secrete an enzyme called hemetin, which dissolves blood clots (handy!).
Pharyngobdellida - no jaws or teeth - they just swallow things whole, eating mall invertebrates (supposed to be small invertebrates, but I liked the typo better).
Leeches can ingest several times their weight in blood and can survive several months without feeding (again, how handy!).
They have one or more pair of eyes. They have sensory organs on their heads and bodies that detect movement, and fluctuations in light and temperature. They also have chemical receptors on the head that allow them to smell.
FOR MATURE READERS ONLY: Leeches are hermaphrodites, with both male and female sex organs. This perhaps will be interesting when Oliver brings it up in preschool. They also have something called a clitellum (we skipped this part), an area of thickened skin that is only visible during mating (hmmmm...). They reproduce by intertwining their bodies and depositing sperm into each other’s clitellar area (speed-breeding?) - good thing we don't have that capacity. After fertilization, the eggs are in a cocoon, which the leech deposits under a log, on a leaf, or maybe in your daughter's ponytail. They hatch several weeks or months later (oh come ON, is it weeks, or is it months???), mini-leeches. They die after reproducing once or twice (I can relate).