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- 



rostra) or straight (Belem- 



nitidae). That these were 



internal or partly internal 



is shown by the fact that 



the chambered part of 



the shell or phragmocone 



is covered by a calcareous 



layer, often laminated, 



which forms the rostrum 



or guard. In the Belem- 



nites the wall of the 



phragmocone (conatheca) 



is continued forwards into 

 ii proostracum, which must have been 

 somewhere in the neighbourhood of the 

 animal's body and head. 



In all living Dibranchs except Spirilla 

 the shell is quite internal, being con- 

 tained in a sac in the anterior wall of 

 the visceral sac, and much reduced. 

 In the Sepiidae, or cuttle-fishes, it is 

 called the cuttle-lone 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 conchyolin, without calcareous matter, and is called the pen. 

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



Fio. 339. Spirula 

 peronii (Bronn). 



FIG. 340. Belemnites . with the re- 

 mains of the body of the animal 

 (after Huxley), a arms with 

 hooks ; 6 head ; c ink-bag ; d 

 phragmocone ; e guard. 



