762 



ZOOLOGY 



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 com- 

 position, between which are interspaces contain- 

 ing 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 

 numerous contractile pigment-containing cells or 

 chromatophores (Fig. 655) situated in the 

 deeper layers of the integument over the entire 

 surface. The chromatophores are flattened sacs 

 with elastic walls, the contracting tendency of 

 which is capable of being counteracted by the 

 action of bundles of muscular fibres radiating 

 outwards from the edge of the sac into the sur- 

 rounding tissues. When these radiating fibres 

 are in action the edge of the chromatophore is 

 drawn outwards in different directions, and as a 

 result the flattened sac becomes more expanded and thinner, 

 the pigment being spread out into a thinner layer. ' When the 

 fibres are relaxed the elasticity of the wall comes into play, and 

 the chromatophore contracts, 



Fl(i. (554.— Shell of 

 Sepia cultrata, 

 posterior view. Re- 

 duced. 



the contained pigment re- 

 suming its former arrange- 

 ment. A peculiar iridescence 

 which, in addition to the 

 play of colours, is recognisable 

 in the integument of Sepia, 

 is due to the presence of a 

 number of cells, the iridocysts. 

 When the mantle-cavity 

 is laid open (Fig. 658) there 

 is seen on each side of it one 

 of the two . plume-shaped 

 ctenidia {den.). In the middle 

 line of the posterior surface, 

 close to the internal opening 

 of the funnel, is the anal 

 aperture (an.) situated at the 

 oral extremity of a longi- 

 tudinal tube — the rectum. 



retd.mus 



Fig. 655.— Chromatophore of Sepia, magnified. 

 nuc. nuclei in wall of sac ; pigm. pigment ; 

 rod. mus. radiating strands of muscle. (After 

 Vogt and Jung.) 



On either side of the rectum is a 



much narrower projecting tube with a terminal opening — the 



