XII 



PHYLUM MOLLUSC A 



711 



Fie. HIT. Shell ut 



Sepia cultrata, 



posterior view. 

 Reduced. 



nerves which supply it, to be, like the arms, a specially modified 

 part of the foot: it corresponds, perhaps, to the 

 metapodium of Triton. 



Fringing each lateral margin of the body is a 

 thin muscular fold the Jin which is used as a 

 swimming organ. 



The anterior wall of the body exhibits, as 

 already mentioned, a hard and resistent char- 

 acter owing to the presence of the internal 

 shell (Fig. 617). This is completely enclosed 

 in a sac of the mantle. Like the body itself, 

 it is bilaterally symmetrical. In shape it may 

 be described as leaf-like, with a rounded and 

 comparatively broad oral end, and a narrower 

 aboral, provided with a sharp, anteriorly-project- 

 ing spine. The posterior surface is convex, the 

 anterior convex towards its oral end, but deeply 

 concave aborally, and bounded laterally by thin 

 prominent wing-like ridges which converge to 

 meet at the aboral extremity. The main mass 

 of the shell consists of numerous, closely- 

 arranged, thin laminae of calcareous composition, 

 between which are interspaces containing gas. 

 On the surface is a thin layer of chitinoid material, and slightly 

 thicker strips of similar composition run along the margins. 



The living Cuttle-fish will be observed to undergo frequent 



changes of colour, and 

 blushes of different hues are 

 to be observed passing over 

 the surface. These are due 

 to the presence of numer- 

 ous contractile pigment-con- 

 taining cells or chromato 

 phores (Fig. 618)' situated 

 in the deeper layers of the 

 integument over the entire 

 surface. The chromatophores 

 have elastic walls, the con- 

 tracting tendency of which 

 is capable of being counter- 

 acted by the action of bundles 

 of muscular fibres radiating 

 outwards from the wall of 

 the sac into the surrounding- 

 tissues. When these radiat- 

 ing fibres are in action the wall of the chromatophore is drawn 

 outwards in different directions, and as a result its cavity is dilated, 



Fn;. t'.is. Chromatophore of Sepia, inagnitiu<l. 

 iii"'. nuclei in wall of sac; pi<jni. pigment; 

 ,-il. IIIKX. radiating strands of muscle. (After 

 Vogt and Jung.) 



