Mr. GARNER on the Nervous System of Molluscous Animals. 491 
plying the mantle (C.). The branchial ganglion (H.), situated as in Aplysia, 
does not send its filament ( f.) as a distinct nerve up to the brain, but it passes 
through the ganglion supplying the mantle. The two cerebral ganglia (A.) 
are here lateral. The pedal ganglia are connected both with the sentient 
lobes, and with those supplying the mantle, as will be found generally the 
case. A subcesophageal nerve completes the ring, and combines the pairs of 
ganglia; and the cerebral ganglia as usual give the nerves forming the pha- 
ryngeal ganglia. | 
The spiral Gasteropoda present considerable variety in their nervous system. 
It may be premised that in them we shall (with a few exceptions, where we 
only find two,) observe four nerves originating from the superior lobes, when 
the ganglia of the ring remain far separate, but from the posterior part of the 
inferior expanded portion of the ring in the higher Gasteropoda. The two 
external ones are the nerves of the mantle, analogous to those we shall see in 
the Sepia (and it is to be remembered that the mantle is an important part in 
respiration) : the two internal ones are analogous to the branchio-visceral 
ones of the same animal. The branchize are sometimes supplied by one, 
sometimes by the other, and sometimes by both; but in the higher Gastero- 
poda entirely by the latter, as in Cephalopoda; and we shall therefore call the 
internal pair, arising from the posterior point of the inferior portion of the 
brain in the higher Gasteropoda, the branchio-visceral when the animal is 
aquatic, or pneumogastric when terrestrial. From these latter, filaments 
go to the viscera, and often form a ganglion at or near the stomach. The 
nerves of the mantle originate external to the preceding, and sometimes wholly 
or in part supply the branchiz. From the shape of the spiral Gasteropoda 
these are more or less twisted in their course. They may be well termed ex- 
ternal respiratory, as they supply the mantle, siphons, and roof and floor of 
the respiratory sac, and often the branchiz more or less. The shell-muscles 
partly receive their nerves from one of these pairs, partly from the pedal 
ganglion. , 
The nervous system of Janthina (fig. 7.) is one of the most simple of those of | 
the spiral Gasteropoda. .In it we have a lateral ganglion (A.) on each side, 
considerably removed from each other, giving origin to the nerves of the eyes 
and tentacles (g.) and lips (i.), and posteriorly each ganglion sends off three 
3s2 
