In Julius Caesar, Act II, who proposes the murder of Antony and why does Brutus oppose it?

The answer to this question can be found in Act II scene 1
of this excellent tragedy. The conspirators are gathered together, and they are working
out a list of those who need to be killed in order to ensure that their assassination of
Caesar and the usurpation of power is successful. It is Cassius who suggests that Antony
should not "outlive Caesar," and suggests, quite accurately as it turns out, that he
would be a "shrewd contriver" who could act against them. It is thus that he argues that
the conspirators should "Let Antony and Caesar fall
together."

However, Brutus argues against this, saying that
to kill Antony in addition to Caesar would be equivalent to cutting the head off a
corpse and then hacking the limbs. He says that Mark Antony is "but a limb of Caesar"
and thus once Caesar himself is killed will be
harmless:

And
for Mark Antony, think not of him;

For he can do no more
than Caesar's arm

When Caesar's head is
off.