THE CEPHALOPODA 297 



the Dibranchia, chromatopliores 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 mesddermic 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 cliiefly 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, Hisliopsis, Pteri/fjioteuthis, etc., 

 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 Pterygioteuthix. 



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.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 



