NERVOUS SYSTEM. 405 
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 ysed 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 Sepa 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, 
foilowed 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. 218), 
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 
short double commissures with the pedals and pleuro-viscerals on the 
ventral side of the gullet. The pedal ganglia at each side are in part 
divided into two,—one half forming the brachial ganglion which sends 
nerves to the arms, the other the infundibular which supplies the 
funnel. 
