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 XXI., 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 (#), 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” (f). 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 
