426 



MOLLUSCA. 



the female Argonaut, which has already been referred to, is not 

 a mantle-shell. Fins of various forms are often present as lateral 

 expansions of the mantle. 



The dermis contains the remarkable chromatophores which cause 

 the well-known play of colours. These consist of large cells filled 

 with pigment (red, blue, yellow, or dark colours) ; to their walls, 

 which are formed of a cellular membrane, numerous radiating 

 muscular fibres (by some observers said to be connective tissue 

 fibres) are attached. When the latter contract the cells are dilated 

 and the pigment spreads over a larger area, and so gives colour to the 



skin. When the contrac- 

 tion ceases the cell returns, 

 in virtue of the elasticity 

 of its walls, to its original 

 shape, and the pigment is 

 again concentrated in a 

 small space, arid the skin 

 becomes uncoloured ; thus 

 the animal changes its 

 colour. The chromato- 

 phores are probably under 

 the control of the will, and 

 are connected with a special 

 centre in the stalk of the 

 optic ganglion. The eye 

 seems to be the organ most 

 intimately connected with 



as to show the chambered shell and the laminated them, for if the Optic nerve 

 guard deposited upon its surface. D, shell of Spirilla . 



i aevis . be cut the power of volun- 



tarily changing colour on 



that side is said to be lost. Nevertheless there seem to be peripheral 

 centres through which these organs* can be brought into action, 

 and the animals seem to have the power of changing their colour 

 involuntarily according to the colour of their environment. In 

 addition to the chromatophores there is a deeper layer of small 

 shining spangles, which produce interference colours and thus give 

 rise to the peculiar lustre and iridescence of the skin. 



In certain abyssal Cephalopods there are cutaneous phosphorescent organs, 

 consisting of a superficial refractile structure and a deep photogenic layer ; they 



* Krukenberg, Vergl. physiol. Studien an den Kusten der Adria, Heidelberg,. 

 1880. 



FIG. 341. Internal shells of Cephalopoda (from Lan- 

 kester). A , shell of Conoteuthis dupininna from the 

 Neocomian of France. />, shell of Sepia Orbigniana, 

 Mediterranean. C, shell of Spiritlirostra Bellardii, 

 from the Miocene of Turin ; the specimen is cut so 



