The octopus, by the way, uses his feelers either as feet or hands;
with the two which stand over his mouth he draws in food, and the last
of his feelers he employs in the act of copulation; and this last one,
by the way, is extremely sharp, is exceptional as being of a whitish
colour, and at its extremity is bifurcate; that is to say, it has an
additional something on the rachis, and by rachis is meant the
smooth surface or edge of the arm on the far side from the suckers.
(See diagram.)

In front of the sac and over the feelers they have a hollow
tube, by means of which they discharge any sea-water that they may
have taken into the sac of the body in the act of receiving food by
the mouth. They can shift the tube from side to side, and by means
of it they discharge the black liquid peculiar to the animal.