414 PHYLUM MOLLUSCA, 



Ammonites. Most of the modern forms seem to be 

 more active than their ancestors, and their shells have 

 degenerated. 



While the fact of the degeneration is perfectly obvious, the line along 

 which it has taken place is difficult and still debated. In Nautilus, 

 although the animal lives within the shell, the mantle fold is for some 

 distance reflected over it ; in the other series of Cephalopods this 

 process has gone further, and, where a shell is present, it is entirely 

 enclosed within the mantle fold, and is much reduced in size. In the 

 extinct Belemnites the internal shell was straight and chambered, but 

 almost concealed by secondary deposits of lime, secreted by the walls of 

 the shell-sac. In Sepia, according to one view, the central laminated 

 region of the " bone" represents the remains of the chambered shell; 

 the remainder corresponds to the secondary deposits of lime in the 

 Belemnites. In Loligo there is no deposit of lime, an organic chitinous 

 pen only being left. In Octopus there is no trace of shell at all. 

 According to some, the shell-sac, in which the shell or pen of 

 Cephalopods is formed, is to be regarded as equivalent to the embryonic 

 shell-sac plus a mantle pocket. 



