378 MOLLUSC A. 



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 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 unmistakable " stellate " 

 ganglion. Removing the skin as carefully as possible over 

 the whole visceral region between the gills, and taking pre- 

 cautions 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 arrange- 

 ment of these organs, we can follow the food canal with its 

 stomach, digestive gland, &c. 



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. 118). 



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 - 

 pharnygeal" ganglion. The cerebral ganglia are also connected by 

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

 ventral side of the gullet. 



The following chief nerves are given off from the central system : 



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



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

 optic ganglion at the base of each eye. 



(2) Ten nerves to the "arms" are given off by the pedal ganglion, 



and this is one of the reasons which have led most morpho- 

 logists to regard these arms as portions of the ' ' foot. " 



(3) Two large nerves from the more ventral portion of the pleuro- 



visceral ganglia form a visceral loop, and give off many 

 branches to the gills and other organs. From the pleural 

 portion arise two mantle nerves, each of which ends in a 

 large stellate ganglion. 



