358 MOLLUSC A. 



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. 



On the shore one often finds the " cuttle bone " or sepio- 

 staire, which is sometimes given to cage birds to peck at 

 for lime, or used for polishing and other purposes. It lies 

 on the dorsal side of the animal, covered over by the mantle 

 sac. In outline it is somewhat ellipsoidal, thinned at the 

 edges like a flint axe-head, and with curved markings which 

 indicate lines of growth. In the very young Septa it con- 

 sists wholly of the organic basis conchiolin, but to this lime 

 is added from the walls of the sac. Between the plates 

 of lime there is gas, and though the structure may give the 

 cuttle some stability, it is probably of more use as a float. 



Internal appearance. — When the mantle flap is cut open 

 and reflected, the two plume-like gills are seen, and the 

 lower end of the siphon. The dark outline of the ink-bag, 

 followed along towards the head, leads our eyes to the end 

 of the food canal. Near this are the external apertures of 

 the two kidneys and of the genital duct. On each side of 

 the base of the funnel lies a very large and unmistakable 

 " stellate " ganglion. Removing the skin as carefully as 

 possible over the whole visceral region between the gills, 

 and taking precautions not to burst the ink-sac, we see the 

 median heart, the saccular kidneys, contractile structures or 

 branchial hearts at the base of each gill, and the essential 

 reproductive organs near the apex of the visceral mass. 

 Disturbing the arrangement of these organs, we can follow 

 the food canal, with its stomach, digestive gland, etc. 



Nervous system. — Three pairs of ganglia surround the 

 gullet, — cerebral on the dorsal and anterior side, pedal and 

 pleuro-visceral on the ventral and posterior side (Fig. 152), 

 but lying so close together that their boundaries are defined 

 with difficulty. All are well protected by the investing 

 cartilages. 



The cerebral ganglia are three-lobed, and are connected anteriorly by 

 two commissures with a " supra- pharyngeal " ganglion, which gives off 

 nerves to the mouth and lips, and is connected also with an " infra- 

 pharyngeal " ganglion. The cerebral ganglia are also connected by 



