36 THE LIFE OF THE MOLLUSCA 



The mouth lies in the centre of these arms, and 

 is furnished with a pair of powerful chitinous jaws 

 that resemble a parrot's beak (Plate III., Fig. 4) ; 

 while the radula, though proportionately small, is 

 well developed (Plate III., Fig. 20). 



The symmetrical body is surrounded by the bell- 

 shaped mantle, covered in some cases by a shell, 

 while those without an external shell frequently have 

 lateral fins. 



On the ventral side a cavity is left between the 

 mantle and the body (Plate XXL, Fig. 3, m.c). In 

 this "mantle cavity" the gills (g) are placed, and 

 into it the termination of the reflexed alimentary 

 canal opens, as well as the ink-sac (i), with which 

 most members of the class are provided. 



Just at the mouth of the mantle cavity two lobes 

 of the skin above the foot, sometimes free and some- 

 times united at their margins, form a " funnel " or 

 " siphon " (/). Through this siphon the water is 

 discharged from the mantle cavity ; quietly during 

 ordinary respiration, or with great force when the 

 creature propels itself backwards through the sea by 

 means of the ejected stream. Through the same 

 orifice the animal can at will eject the inky fluid 

 from the ink-sac, with which it clouds the water on 

 emergency when seeking to escape its foes. This 

 power is possessed by all the living Cephalopoda 

 save Nautilus, Cirroteuthis, and two species of Polypus. 

 An ink-sac was present in the fossil Belemnites, and 



