MR. T. H. HUXLEY ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA. 53 
These three pairs of primary ganglia are the Cephalic, the Pedal, and the Parieto- 
splanchnic, 
I. The Cephalic Ganglia. — These are always either in apposition, or are united by 
a commissure above the oesophagus. They give off either immediately or from the 
connecting commissure, the following nerves : — 
1. Labial, to the lips and anterior parts of the head. 
2. Olfactory, to the tentacles. 
3. Optic. 
4. Buccal, to the buccal mass, tongue, and jaws. 
Accessory ganglia may be developed upon all these nerves. They are found upon 
the labial nerves in Gastei'opteron, three upon each side (Souleyet and Blanchard) ; 
upon the olfactory nerves in many Nudibranchiata (Alder and ILancock, Souleyet, 
&c.) ; upon the optic nerves in Cephalopoda and Heteropoda. 
The presence of ganglia upon the buccal nerves is almost constant. There seems 
to be only one inferior buccal ganglion in some Cephalopoda, while in others there 
are two, one above and one below. In the Heteropoda, Pteropoda, and most Gas- 
teropoda, there is a pair of ganglia placed laterally at the re-entering angle of the 
oesophagus and buccal mass. In Patella, Haliotis, and Fissurella, I have found 
four, two in the latter position, and two anteriorly, just where the buccal nerves 
come off. 
The buccal ganglia are always united by a commissure, so that when the cerebral 
ganglia are above the oesophagus, an anterior nervous ring is formed ; when they 
are at the side or below, as in Pteropoda, there is no anterior nervous ring. 
II. The Pedal Ganglia. — These are either in contact or are united by a commissure ; 
below the oesophagus they give off— 
1. Auditory nerves. 
2. Pedal nerves. 
The auditory nerves are not commonly present, as their organs are generally 
sessile; however, they exist in Cephalopoda, and in Stromhus CLud Pteroceras. As 
has been stated above, the Heteropoda make an extraordinary exception to the 
usual position of the auditory organs, since in them these nerves appear to be given 
off from the cephalic ganglia. Considering, however, that the auditory nerves are 
invariably attached to the pedal ganglia in all other Mollusks, and that in Pteroceras 
and Stromhus, genera which so nearly approach the Heteropoda, the auditory nerves 
are very long, I do not thirdv it very hazardous to suppose, that in the Heteropoda 
the auditory nerves really proceed from the pedal ganglia, but have become united 
to the cephalic ganglia. 
In any other case their position is quite exceptional, for the supraffisophageal 
position of the auditory sacs in Nudibranchiatata merely arises from the pedal 
ganglia being thrust upwards, and united with the cephalic ganglia. 
The accessory ganglia of the pedal ganglion appear to be only what may be called 
MDCCCLIII. I 
