340 



:MOLLrSCA 



flaltcncd and tlius inlluence tlic color; when the fibres relax Ihe pii,mient cells 

 contract to small spots. In deep-sea cephalopods phosphorescent organs ha\e 

 been observed. 



Xotwilhslaiiding the soft bodies a well-developed shell occurs in 

 living cephalopods only in Xaiilihis and Ari;oi!aiild (tigs. 352, 303). 



Fig. s^5. — Spirilla, with internal shell val'lcr C)\ven). 



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

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

 partitions into numerous chambers wliicli increase in size towards the 

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



Fig. 354. — Diapiranr of shells, etc. of various cephalo|Kiils (after T.ansijV .1, 

 Scfiia; B, hfloscpia; C, Pi-lrmiti/rs-: /), 0.';ti;it-(>lrii/liis; A', OiiiDin'ilrrphes. 0, anterior; 

 p, posterior; />//, phragmocoiic; pr, proostraeuni; r, rostrum; .v, siphon. 



towards the centre of the shell {cxof;aslric positioit) ; the other cham- 

 bers are tilled with air. luicli partition lias a small opening, and 

 through these runs a stand of tissue, the siphuudc. yVmong the fossil 

 cephalopods many forms — lire Nautiloids and Ammonites — ha\-o similar 



