THE CEPAALOPODA 297 
the Dibranchia, chromatophores or extensible pigment cells, whose 
activity produces the remarkable colour changes characteristic of 
these animals. The chromatophores are cells originally of ecto- 
dermic origin, which sink below the epithelium and become 
connected with contractile radiating mesodermic fibres. The 
pigment cells are simple but multinuclear, since they contain 
secondary nuclei situated at the bases of the muscular fibres. 
Different cells contain different coloured pigment: yellow, brown, 
red, or blue in the Decapoda. Each cell exhibits a constant though 
feeble tremulous movement, and may suddenly be extended, by a 
reflex action, under the influence of emotion or excitation, or as a 
more direct result of volition the chromatophores of the same colour 
may assume a definite condition of contraction or expansion, which 
gives the body a tint analogous to that of surrounding objects. In 
the latter case the action of the chromatophores is under the direct 
influence of the cerebral centres of the nervous system, and section 
of one of the optic nerves puts an end to voluntary changes of colour 
on the same side of the body. The chromatophores are chiefly dis- 
tributed over the anterior surface (or upper surface when the animal 
is in its natural position) of the mantle, the head, and the external 
sides of the arms. In the Decapoda there is, in addition to the 
chromatophores, a layer of reflecting cells which give these animals 
their iridescent hues. 
In certain Oigopsida belonging to the zonary or deep plankton, 
for example, Histioteuthis, Calliteuthis, Histiopsis, Pterygioteuthis, ete., 
the surface of the body bears luminous organs, all of which are 
oriented towards the anterior extremity. The essential structure 
of these organs consists of a deeper photogenous layer and of super- 
ficial refracting elements. They may even extend into the interior 
of the pallial cavity, as, for example, in Pterygioteuthis. 
In the deeper parts of the integument the connective tissue is 
often concentrated to form cartilage of analogous structure to that 
of the Vertebrates, but characterised by the existence of processes 
of the cartilage cells which ramify in the matrix and anastomose 
with one another (Fig. 265). This cartilage is specially well de- 
veloped in the head. In Nautilus there is a capito-pedal cartilage, 
shaped like a letter H (Fig. 264, A), which only supports the ventral 
part of, the nerve-centres, two of its branches extending into the base 
of the funnel. In the Dibranchia the cephalic cartilage completely 
encloses the central nervous system and the otocysts and is traversed 
by the oesophagus (Fig. 268, ¢.c). In some cases the cephalic car- 
tilage is produced into anterior expansions ; such are the pre-orbital 
cartilages surrounding the eyes of Sepia (Fig. 264, C). Various 
muscles, notably the retractor muscles of the head, take their origin 
from this “cranial” cartilage. There are also cartilaginous skeletal 
elements in other parts of the body of various Cephalopoda. In 
