_Histioteuthis, Calliteuthis, Histiopsis, Pterygioteuthis_, the
surface of the skin bears photogenous organs directed towards the oral
extremity. Anatomically these consist of a deeper photogenous layer
and a more superficial refracting layer. In some cases, e.g.
_Pterygioteuthis_, they occur even within the mantle-cavity.

_Fins._--In the majority of the Decapoda and in the _Cirrhoteuthidae_,
the mantle is produced into lateral symmetrical expansions which have
the function of fins. They originate at the aboral extremity where
they remain in _Spirula_ (fig. 18). In most other Oigopsida they are
terminal, but more dorsal than ventral, e.g. _Loligopsis_ (fig. 16),
and there may be two on each side, as in _Grimalditeuthis_. In other
cases they extend laterally along a greater length of the body, as in
_Sepia_ (fig. 15). In _Ctenopteryx_ they have a superficial
resemblance to the fins of fishes, consisting of a thin membrane
supported by a series of muscular rods.

_Chromatophores._--These are characteristic of the Dibranchiata,
apparently absent in _Nautilus_. They are originally single cells of
ectodermic origin which sink below the epidermis and become connected
with radiating muscular fibres. The cells are single but multinuclear.
Different cells contain pigments of different colours, yellow, brown,
red or blue. Each cell in life is in constant tremulous movement;
under the influence of nervous excitement the cells are suddenly
expanded or contracted, producing blushes of colour and pallor. By
reflex action of which the afferent stimulus acts upon the eyes as in
fishes, the chromatophores assume a condition which approximates the
colour of the animal to that of surrounding objects. In the Decapoda
there are also reflecting elements which produce iridescent hues.

_Aquiferous Cavities._--In addition to the pockets into which the
tentacular arms of Decapoda are retracted, there are in several
Dibranchiata cavities in the integument which open to the exterior by
special pores but have no communication with the vascular system or
other internal cavities of the body. In _Ocythoe_ there are such pores
on the back of the head and at the base of the funnel; buccal pouches
on the ventral side of the mouth, internal to the arms, occur in some
genera, one in _Loligo_, two in _Sepia_. In some species of _Sepia_
there are pouches in the mantle.

_Alimenta