XII 



PHYLUM MOLLUSCA 



711 



FIG. 617. Shell of 

 Sepia cultrata, 



posterior 

 Reduced. 



view. 



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 fin which is used as a 



swimming organ. 



The anterior wall of the body exhibits, -as 



a! remedy 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 



aiiterioxjcojiy_eJL 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, tkhr^mrrrme-oi^calcareous composition, 



between- which are interspaces containing gas. 



On the surface is a thin layer of chitinoid material, 



thicker strips of similar composition run along the margins. 



Oattle-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 



FIG. 618. Chromatophore of Sepia, magnified. of UlUSCUlar fibres radiating 



nuc. nuclei in wall of sac: pir/m. pigment; ^i+^royrlo frwm t>io wall nf 



,-,,!. , ,<*., - :l dititiiiK strands of muscle. (After OUtWaiClS trOlYL tne Wall O 



Yogt and Jung.) ^ ne gac i n to the SUTTOUnding 



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, 



rael.Tmis 



