CEPHALOPODA. 



59 



flfxi 



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 (Argonauta, 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 (si2rfion), which perforates the 

 partitions an4 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- 



dsp 



fibres are attached. When the latter 

 contract the cells are pulled out into 



Gsp, splanchnic ganglion ; M, stomach ; 

 M', blind appendage of stomach ; A, 

 anus ; Tb, ink sac. 



bmne, numerous radiating muscular FIO. 532. Digestive apparatus or sepia 



(after W. Keferstain). L, Up ; Mxi, Mxs, 

 lower and upper jaws ; Ra, radula ; Jig, 

 buccal ganglion ; Spd, salivary gland ; 

 , , Oe, oasophagus ; L, liver ; Gg, bile duct ; 



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 



