SHELLS OF CEPHALOPODS 



487 



The Nautiloids began in the Cambrian and died out at the end of 

 the Palaeozoic period, except the Orthoceras and Nautilus-\i\i.& types. 

 The genus Nautilus appeared in the 

 Cretaceous. The Ammonite series 

 lasted from the Devonian to the close 

 of the Cretaceous. The two series 

 show a frequent parallelism in archi- 

 tecture. 



The Cephalopods are the most 

 speciahsed of the Molluscs, and 

 present much diversity of type. 

 They swim freely in the sea, or 

 creep sluggishly among the rocks. 

 They are voracious eaters, and 

 devour very diverse kinds of 

 animals, their parrot-like jaws and 

 powerful odontophore, as well as 

 the numerous suckers, rendering 

 them formidable adversaries. 

 Many live at considerable depths, 

 and their chief foes are the toothed 

 whales, some of which, Uke the 

 sperm whale (Physeter) and the 

 bottle-nose (Hyperoodon), subsist 

 almost entirely on cuttles. Some 

 deep-sea forms have highly evolved 

 luminous organs. 



Shells of Cephalopods. — A cham- 

 bered external shell, serving as a house, 

 is present in Nautilus alone among living 

 Cephalopods. Most of the extinct forms 

 had large and efficient shells of very- 

 diverse shape, some straight like Ortho- 

 ceras, or coiled, with chambers separated Fig. 277. — Spirula, a small 

 by complex septa, as in the Ammonites. Decapod cuttlefish. 



In the majority of shells of the Am- jhe mantle (M.) has been partly 

 monitid series, the septa between the 

 chambers are convex towards the 

 aperture (the opposite in the Nautilus) ; 

 the siphuncle is marginal or ventral ; the 

 septal necks of the siphuncle project 

 forwards (not backwards as in the Nautilus) ; there is an 

 initial chamber or protoconch at the apex of the spiral (per- 

 haps represented by a cicatrix in the Nautilus) ; the suture lines 



cut away to show the spiral 

 chambered shell (SH. ),which has 

 become internal. L., A lumin- 

 escent organ ; F. a terminal 

 fin. 



