404 



PHYLUM MOLLUSC A. 



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 by a 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 

 giant cuttlefish is 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 

 valve within the siphon 

 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"; (b) 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. 



FIG. 217. External appearance ot 

 a cuttlefish (Loligo}. 



