ovided with an
internal valve projecting from its somatic surface, which allows water
to pass outwards but prevents it passing inwards. The mantle performs
rhythmical respiratory movements of expansion and contraction, the
water entering between funnel and mantle and passing out through the
funnel. In Decapoda the edge of the mantle bears internally on each
side a cartilaginous projection which fits into a corresponding
depression on the external surface of the funnel; this is called the
"resisting apparatus," and serves to make the union of mantle and
funnel firmer during expiration. More powerful expiratory movements
are used for sudden retrograde locomotion through the water.

[Illustration: FIG. 26.--Diagram representing a vertical approximately
median antero-posterior section of _Sepia officinalis_ (from a drawing
by A.G. Bourne). The lettering corresponds with that of fig. 10, with
which this drawing is intended to be compared.

a, Shell (here enclosed by a growth of the mantle).

b, The nuchal plate (here a cartilage).

c, (The reference line should be continued through the black area
representing the shell to the outline below it), the integument
covering the visceral hump.

d, The reflected portion of the mantle-skirt forming the sac which
encloses the shell.

e, The inferior margin of the mantle-skirt (mouth of the pallial
chamber).

f, The pallial chamber.

g, The vertically cut median portion of the siphon.

i, The valve of the siphon.

m, The two upper lobes of the fore-foot.

n, The long prehensile arms of the same.

o, The fifth or lowermost lobe of the fore-foot.

p, The third lobe of the fore-foot.

q, The buccal membrane.

v, The upper beak or jaw. s, The lower beak or jaw.

t, The lingual ribbon.

x, The viscero-pericardial sac.

n.c, The nerve-collar.

cr, The crop.

gizz, The gizzard.

an, The anus.

c.t, The left ctenidium or gill-plume.

vent, Ventricle of the heart.

a.b.v, Afferent branchial vessel.

e.b.v, Efferent branchial vessel.

re, Renal glandular mass.

n.n.a, Left nephridial aperture.

visc.per.apert, Viscero-pericardial aperture (see fig. 29).

br.b, Branchial heart.

app, Appendage of the same.

i.s, Ink-bag.]

_Luminous Organs._--In certain Oigopsida living in deep water, e.g.