xn 



PHYLUM MOLLUSCA 



741 



shell (Fig. 663). 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 end, provided 

 with a sharp, anteriorly-projecting 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 numerous contractile pig- 

 ment - containing cells or 

 chromatophores (Fig. 664) 

 situated in the deeper layers 

 of the integument over the 

 entire surface. The chromato- 

 phores 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, the 

 contained pigment resuming its former arrangement. 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. 667) there is seen 

 on each side of it one of the two plume-shaped ctenidia (cten.). 

 In the middle line of the posterior surface, close to the internal open- 

 ing of the funnel, is the anal aperture (an.) situated at the oral 

 extremity of a longitudinal tube the rectum. On either side of 

 the rectum is a much narrower projecting tube with a terminal 



771 



Fia. 664. Chromatophore of Sepia, magnified. 

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

 rod. mus. radiating strands of muscle. (After 

 Vogt and Jung.) 



