Any animal that sucks blood can be called a blood sucker. That includes leeches as well as mosquitoes, ticks, bedbugs, conenose bugs, chiggers, lampreys, and many others. I wouldn't call a Vampire Bat a blood sucker because it laps blood rather than sucking it.
Leeches (Class Hirudinea) are segmented worms, in the same phylum (Annelida) as earthworms.
