A leech who, having penetrated the shell of a turtle only to find that the creature has long been dead, deems it expedient to form a new attachment to a fresh turtle.
EVANGELIST, n. A bearer of good tidings, particularly (in a religious sense) such as assure us of our own salvation and the damnation of our neighbors.
PROBOSCIS, n. The rudimentary organ of an elephant which serves him in place of the knife-and-fork that Evolution has as yet denied him. For purposes of humor it is popularly called a trunk.
SIREN, n. One of several musical prodigies famous for a vain attempt to dissuade Odysseus from a life on the ocean wave. Figuratively, any lady of splendid promise, dissembled purpose and disappointing performance.