334 MOLLUSCS. 



peck at for lime, and is used as a polishing powder, as 

 pounce, as a dentifrice and cosmetic. It lies on the convex 

 and more coloured side of the animal, covered over by skin. 

 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 Sepia, it consists 

 wholly of the organic basis conchiolin, but this is only 

 obvious at the edges of the fully formed structure which 

 contains in addition lime and gas. It has a very spongy 

 texture, and though it may give the cuttle some backbone, it 

 is probably of more use as a float. 



Internal Appearance. — When we cut open the mantle flap 

 and fold the halves back, we at once see the two plume-like 

 gills, 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 

 unmistakeable " stellate " ganglion. Removing the skin as 

 carefully as possible over the wholfe 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, liver, pancreas, 

 etc. 



The Nervous System is concentrated in three pairs of 

 ganglia which surround the gullet,— cerebral on the dorsal 

 and anterior side, pedal and pleuro-visceral on the ventral 

 and posterior side. 



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-pharyn- 

 gea ganglion. Secondly, the cerebral ganglia are connected by short 

 double commissures, with the pedals and pleuro-viscerals on the other 

 side of the gullet. 



'^']^/°i'u"'™^ chief nerves are given off from the central system •— 

 (I) The very thick optic nerves are given off from the commissures 

 between cerebrals and pleuro-viscerals, and lead to a laree 

 optic ganghon at the base of each eye. 



