w of the postero-ventral surface of a male
_Sepia_, obtained by cutting longitudinally the firm mantle-skirt and
drawing the divided halves apart. This figure is strictly comparable
with fig. 4. (From Gegenbaur.)

C, The head.

J, The mid-foot or siphon, which has been cut open so as to display
the valve i.

R, The glandular tissue of the left nephridium or renal-sac, which
has been cut open (see fig. 29).

P, P, The lateral fins of the mantle-skirt.

Br, The single pair of branchiae (ctenidia).

a, The anus--immediately below it is the opening of the ink-bag.

c, Cartilaginous socket in the siphon to receive c', the
cartilaginous knob of the mantle-skirt--the two constituting the
"pallial hinge apparatus" characteristic of Decapoda, not found in
Octopoda.

g, The azygos genital papilla and aperture.

'i, Valve of the siphon (possibly the rudimentary hind-foot)

m, Muscular band connected with the fore-foot and mid-foot (siphon)
and identical with the muscular mass k in fig. 3.

r, Renal papillae, carrying the apertures of the nephridia.

v.br, Branchial efferent blood-vessel.

v br', Bulbous enlargements of the branchial blood-vessels (see figs
28, 29).

t, Ink-bag]

In the males of the Dibranchiata one of the arms is more or less
modified in connexion with the reproductive function, and is called
the "hectocotylized arm." This name is derived from the condition
assumed by the arm in those cases in which its modification is carried
out to the greatest extent. These cases are those of the Octopods
_Argonauta argo_ and _Ocythoe catenulata_ (fig. 24). In the males of
these the third arm (on the left side in _Argonauta_, on the right
side in _Ocythoe_) is found before the breeding season to be
represented by a globular sac of integument. This sac bursts, and from
it issues an arm larger than its neighbours, having a small sac at its
extremity in _Ocythoe_ (fig. 24. x), from which subsequently a long
filament issues. Before copulation the male charges this arm with the
spermatophores or packets of spermatozoa removed from its generative
orifice beneath the mantle-skirt, and during coitus the arm becomes
detached and is left adhering to the female by means of its suckers. A
new arm is formed at the cicatrix before the next breeding season. The
female, being much larger than the male,