I happen to be reading Dennett’s “Breaking the Spell” at the moment, so refreshing after Dawkins (creator of the “extended-phenotype” term) attempt on the same topic. Dennett opens early in his book with the lancet-fluke & ant example to illustrate the “viral” metaphor of a meme infecting a brain producing behaviour inexplicable in terms of the brain’s host’s interests.
The examples here, collected by Neurophilosophy are fungal / ants (and other arthropods), and worm / arthropod cases – but excellent illustrations, if a little gruesome for the squeamish.