CEPHALOPODA. 



59 



Mxi 



opening of the mantle being firmly closed by the sucker-like arrange- 

 ment at the base of the funnel ; the animal, in consequence of the 

 reaction, is thus projected backwards. 



Many Cephalopoda are naked (Octopoda), others (Decapoda) possess 

 an internal rudimentary shell, a few (Argonaut a, Nautilus) are pro- 

 vided with an external spirally-coiled shell. The internal shell rudiment, 

 of the Decapoda lies in a pocket in 

 the dorsal mantle, and is usually a 

 flat, lancet-shaped spongy calcareous 

 plate (os sepice). The external shell 

 is only exceptionally thin and simple 

 (Argonauta) ; usually it is spirally- 

 twisted and divided by cross partitions 

 into a number of successive chambers. 

 The animal lives in the anterior 

 chamber, which is the last formed 

 and largest. The other chambers, 

 which diminish continuously in size 

 backwards, are filled with air; they 

 remain, however, connected with the 

 large anterior chamber by a central 

 tube (siphon), which perforates the 

 partitions and contains a prolongation 

 of the animal's body. 



The dermis of the Cephalopoda 

 contains the remarkable chromato- 

 phores, which cause the well-known 

 play of colours. These consist of cells 

 filled with pigment; to their walls, 

 which are formed of a cellular mem- 

 brane, numerous radiating muscular FIG. 532. Digestive apparatus of Sepia 



fibres are attached. When the latter 



contract the cells are pulled out into 



a star shape; in the processes so 



formed the pigment is distributed. 



When the contraction ceases, the cell 



returns, in virtue of the elasticity of its walls, to its original 



spherical form and the pigment is again concentrated in a small 



space ; thus the animal changes its colour. There are usually two 



kinds of chromatophores, as far as colour is concerned, placed above 



and near one another. They are connected with a special centre 



(after W. Keferstein). L, lip ; Mxi, Mxs, 

 lower and upper jaws ; Ra, radula ; Eg, 

 buccal ganglion ; Spd, salivary gland ; 

 Oe, oesophagus ; L, liver ; Og, bile duct ; 

 Gsp, splanchnic ganglion ; M, stomach ; 

 M', blind appendage of stomach ; A> 

 anus ; Tb, ink san. 



