340 



MOLLUSC A 



flattened and thus influence the color; when the fibres relax the pigment cells 

 contract to small spots. In deep-sea cephalopods phosphorescent organs have 

 been observed. 



Notwithstanding the soft bodies a well-developed shell occurs in 

 living cephalopods only in Nautilus and Argonauta (figs. 352, 363). 



FIG. 353. Spirula, with internal shell (after Owen). 



Externally the shell of the former, coiled in a plane, resembles that of 

 certain snails like Planorbis; but on section it is seen to be divided by 

 partitions into numerous chambers which increase in size towards the 

 aperture. The animal occupies only the last chamber with its back 



FIG. 354. Diagram of shells, etc. of various cephalopods (after Lang). A, 

 Sepia; B, Belosepia; C, Belemnites; D, O.ttracoteuthix; E, Omniastrephes. a, anterior; 

 p, posterior; pit, phragmocone; pr, proostracum; r, rostrum; 5, siphon. 



towards the centre of the shell (exogastric position) ; the other cham- 

 bers are filled with air. Each partition has a small opening, and 

 through these runs a stand of tissue, the siphnndc. Among the fossil 

 cephalopods many forms the Nautiloids and Ammonites have similar 



