"fish" are a paraphyletic group: all chordates excluding tetrapods.
For a more humorous treatment of this concept...
Mutate and Survive
On two occasions I have been asked, — "Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?" ... I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question. - Charles Babbage