CEPHALOPODA. 



425 



extends on to the convexity of the coil just dorsal to the hood 

 <Fig. 338, Ma). 



In the Dibranchs there is a spiral, chambered, mantle-shell in the 

 Decapod Spirula, but it is coiled in the 

 opposite way to that of Nautilus (endo- 

 gastric), and does not enclose the vis- 

 ceral sac. It is in fact partly internal, 

 being largely covered by lobes of the 

 mantle. The septa are, however, per- 

 forated by a siphon, which contains a 

 prolongation of the visceral sac. 



In certain extinct Dibranchs there 

 was a similar internal, chambered shell 

 either coiled (Spiruli- 



rpstra) or straight (Belem- 



nitidae). That these were 



internal or partly internal 



is shown by the fact that 



the chambered part of 



the shell or pliragmocone 



is covered by a calcareous 



layer, often laminated, 



which forms the rostrum, 



or guard. In the Belem- 



nites the Avail of the 



phragmocone (conatheca) 



is continued forwards into 

 a proostracum, Avhich must have been 

 somewhere in the neighbourhood of the 

 animal's body and head. 



In all living Dibranchs except Spirula 

 the shell is quite -internal, being con- 

 tained in a sac in the anterior Avail of 

 the visceral sac, and much reduced. 

 In the Sepiidae, or cuttle-fishes, it is 

 called the cuttle-hone or sepiostaire, and 

 consists of a broad plate composed of 

 laminated tissue containing air spaces and ending behind in a 

 pointed rostrum. In the Squids it is a lamellar body composed 

 of conchy olin, Avithout calcareous matter, and is called the pen. 

 In the Odopoda both the shell and its sac are absent. The shell of 



-e 



FIG. SiQ.Iitlcmnites with the re- 

 mains of the body of the animiil 

 (after Huxley), a arms with 

 hooks ; b head ; c ink-bag ; </ 

 phragmocone ; c guard. 



