404 PHYLUM MOLLUSCA. 
At one time the mantle cavity is wide, and you can thrust 
your fingers into its gape; when about to contract, this 
gape is closed bya strange double hook-and-eye arrange- 
ment; contraction occurs, and the water, no longer free to 
leave as it entered, gushes out by the funnel, the base of 
which is within the mantle cavity. The suckers on the 
arms are muscular cups, borne’ on little stalks (unstalked 
in Octopus, etc.) well innervated, and able to grip 
with a tenacity which in 
giantcuttlefishis dangerous 
even to men. The inner 
edge of the cup margin is 
supported by a chitinoid 
ring bearing small teeth. 
Each cup acts as a sucker, 
in a fashion which has 
many analogues, for a 
retractor muscle increases 
the size of the cavity 
after the margin has been 
applied to some object. 
The external pressure is 
then greater than that 
within the cup, and the 
little teeth keep the attach- 
ment from slipping. 
It seems likely that the 
arms represent a_ pro- 
podium, and the siphon 
a mesopodium, and a 
Fic. 217.—External appearance of Valve within the siphon 
a cuttlefish (Zo/igo), has been compared to a 
metapodium. 
Skeletal system.—An internal skeleton is represented by 
supporting cartilaginous plates in various parts of the body, 
especially—(a) in the head, round about the brain, arching 
over the eyes, enclosing the “‘ears”; (4) at the bases of the 
arms; (c) as a crescent on the neck; (d) at the hook-and- 
eye arrangement of the mantle flap; (e) along the fringing 
fins. Ramified “stellate” cells lie in the structureless 
transparent matrix of the cartilage. 
